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

+ remove vp10_ prefix

quiets a -Wmissing-prototypes warning

BUG=b/29584271

Change-Id: I8821c38009b90296280f9b14233e73c92076e81f

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