codec: Makefile.am: rename Videotoolbox plugin
Because of how the MODULE_NAME macro is injected into automake from AM_CPPFLAGS and how automake will compile files that have no specific compilation flags from a given target (ie. no _CPPFLAGS override or _CFLAGS/_CXXFLAGS/_OBJCFLAGS override), the videotoolbox module was named "decoder" (as the first compiled file) which is highly confusing to read in the log.
Forcing the default CPPFLAGS will ensure we correctly get videotoolbox as the MODULE_NAME.