Correct a macro
Correct a macro

--enable-lowbitdepth defines the flag CONFIG_LOWBITDEPTH, not
CONFIG_AOM_LOWBITDEPTH.

Change-Id: Ifa1c12847bee4978d08d010f4fc3601d75e59c31

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