vp10/encoder/rd.c: make a function static
vp10/encoder/rd.c: make a function static

+ remove vp10_ prefix

quiets a -Wmissing-prototypes warning

BUG=b/29584271

Change-Id: I6b5d71f8120a6d1fee4c782beb4c6d6eef980f65

mbebenita/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom