fix compilation of test/gl_in_mainthread_after_pthread.c

[esqu1]

The test was missing an Emscripten header so wasn't compiling. Should be working now!

Compiled with:

emcc gl_in_mainthread_after_pthread.c -o hello.html -pthread -sOFFSCREEN_FRAMEBUFFER 

[sbc100]

Hmm, this test is currently disabled:

emscripten/test/test_browser.py

Lines 4311 to 4315 in 4218f89

	@requires_offscreen_canvas
	@requires_graphics_hardware
	@disabled('This test is disabled because current OffscreenCanvas does not allow transfering it after a rendering context has been created for it.')
	def test_webgl_offscreen_canvas_in_mainthread_after_pthread(self, args):
	self.btest('gl_in_mainthread_after_pthread.c', expected='0', emcc_args=args + ['-pthread', '-sPTHREAD_POOL_SIZE=2', '-sOFFSCREENCANVAS_SUPPORT', '-lGL'])

.

I wonder if we can get it working? If not its probably best just to delete it. How did you come across this test BTW?