Fix missing abs declaration on Emscripten

[KivApple]

When building cglm for Emscripten the following error happen:

In file included from C:/Users/kiv/Projects/cglm/src/frustum.c:8:
In file included from C:/Users/kiv/Projects/cglm/src/../include/cglm/cglm.h:15:
C:/Users/kiv/Projects/cglm/src/../include/cglm/ivec2.h:252:13: error: call to undeclared library function 'abs' with type 'int (int)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  dest[0] = abs(v[0]);
            ^
C:/Users/kiv/Projects/cglm/src/../include/cglm/ivec2.h:252:13: note: include the header <stdlib.h> or explicitly provide a declaration for 'abs'
1 error generated.

This pull request fixes this issue.

[recp]

Hi @KivApple, thanks for your contribution[s], the PR is merged 🚀