vp10/common/idct.h: add some missing prototypes
vp10/common/idct.h: add some missing prototypes

quiets the warning of the same name

BUG=b/29584271

Change-Id: I220cd58e1060f77e3910472fed1b167add3a08f8

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