perfcntrs.c: remove include GLES3/gl3.h

Merged Erico Nunes requested to merge enunes/kmscube:perfcntrs-gles2 into master

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.

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