Fixed syntax error of checking defined identifier

EDIT: The original issue has been resolved in the discussion but a new bug has also been found. This pull request is changed to fix the syntax error of checking defined identifier to make the original purpose work as expected.

ORIGINAL TITLE: Fixed duplicate symbols issue
ORIGINAL DESCRIPTION:
The compiler will throw duplicate symbols error If including nanovg_gl_utils.h" or "nanovg_gl.h" in multiple source files.

This commit adds `static` in front of all functions in both header files to prevent from generating the error.