Fix Emscripten build

[Birch-san]

Fixes #855.

encountered this compile error when using build_emscripten.sh:

[ 55%] Building C object src/CMakeFiles/box2d.dir/timer.c.o
cd /Users/birch/git/box2d/build/src && /Users/birch/git/emsdk/upstream/emscripten/emcc -DBOX2D_ENABLE_SIMD @CMakeFiles/box2d.dir/includes_C.rsp -g -std=gnu17 -fvisibility=hidden -ffp-contract=off -msimd128 -msse2 -Werror -MD -MT src/CMakeFiles/box2d.dir/timer.c.o -MF CMakeFiles/box2d.dir/timer.c.o.d -o CMakeFiles/box2d.dir/timer.c.o -c /Users/birch/git/box2d/src/timer.c
/Users/birch/git/box2d/src/timer.c:90:8: error: no member named 'start_sec' in 'struct b2Timer'
   90 |         timer.start_sec = t.tv_sec;
      |         ~~~~~ ^
/Users/birch/git/box2d/src/timer.c:91:8: error: no member named 'start_usec' in 'struct b2Timer'
   91 |         timer.start_usec = t.tv_usec;
      |         ~~~~~ ^
/Users/birch/git/box2d/src/timer.c:99:28: error: no member named 'start_sec' in 'struct b2Timer'
   99 |         time_t start_sec = timer->start_sec;
      |                            ~~~~~  ^
/Users/birch/git/box2d/src/timer.c:100:47: error: no member named 'start_usec' in 'struct b2Timer'
  100 |         suseconds_t start_usec = (suseconds_t)timer->start_usec;
      |                                               ~~~~~  ^
/Users/birch/git/box2d/src/timer.c:123:28: error: no member named 'start_sec' in 'struct b2Timer'
  123 |         time_t start_sec = timer->start_sec;
      |                            ~~~~~  ^
/Users/birch/git/box2d/src/timer.c:124:47: error: no member named 'start_usec' in 'struct b2Timer'
  124 |         suseconds_t start_usec = (suseconds_t)timer->start_usec;
      |                                               ~~~~~  ^
/Users/birch/git/box2d/src/timer.c:141:9: error: no member named 'start_sec' in 'struct b2Timer'
  141 |         timer->start_sec = t.tv_sec;
      |         ~~~~~  ^
/Users/birch/git/box2d/src/timer.c:142:9: error: no member named 'start_usec' in 'struct b2Timer'
  142 |         timer->start_usec = t.tv_usec;
      |         ~~~~~  ^
8 errors generated.
make[2]: *** [src/CMakeFiles/box2d.dir/timer.c.o] Error 1
make[1]: *** [src/CMakeFiles/box2d.dir/all] Error 2
make: *** [all] Error 2

seems like timer.c expects to be able to assign to start_sec, start_usec when __EMSCRIPTEN__ is defined, so I've declared those propeties on b2Timer in that circumstance.

[ErikSom]

Awesome!

[erincatto]

Similar fixes in #852