add pxr/imaging/garch/glApi.h as a GL loader header like glew.h that should be included first

Core USD 21.02 dropped the dependency on GLEW in favor of its own GL loader in
garch.

Author: mattyjams

.clang-format

@@ -43,9 +43,9 @@ IncludeCategories:
   # 8. C system headers
   # 9. Conditional includes
 
-  # 0. Glew must be included before any other GL header
+  # 0. GL loaders must be included before any other GL header
   # Negative priority puts it above the default IncludeIsMainRegex
-  - Regex:           '<pxr/imaging/glf/glew.h>'
+  - Regex:           '<pxr/imaging/(garch/glApi.h|glf/glew.h)>'
     Priority:        -1
 
   # 1. Related header