perfcntrs.c: remove include GLES3/gl3.h
GLES3/gl3.h is not needed to build perfcntrs.c, and the functionality implemented there is also compatible with GLES2 only.
Fixes build in environment with only GLES2 headers.
Closes #9 (closed)
Also !38 (closed) has been open for a while and tried to address it differently, but this should be simpler and also get the issue fixed.