Merge changes from topic 'missing-proto'

* changes:
  vp9_subexp.h: add a missing prototype
  vp9: add some missing includes
  vp9 intrinsics: add vp9_rtcd include
  vp9: correct some function signatures
  vp9_variance_sse2: sync function signatures
  vp9/encoder: make some functions static
  vp9_dct_sse2: make some functions static
  vp9_decodeframe.c: make a function static