build: Drop YASM-OBJS-FFT from SUBDIR_VARS.

FFT code only appears in libavcodec, so there is no need to reset the variable
when the build templates are instantiated for other libraries.

Makefile

@@ -100,7 +100,7 @@ config.h: .config
 	@-tput sgr0 2>/dev/null
 
 SUBDIR_VARS := OBJS FFLIBS CLEANFILES DIRS TESTPROGS EXAMPLES SKIPHEADERS \
-               ALTIVEC-OBJS MMX-OBJS NEON-OBJS YASM-OBJS-FFT YASM-OBJS    \
+               ALTIVEC-OBJS MMX-OBJS NEON-OBJS YASM-OBJS                  \
                HOSTPROGS BUILT_HEADERS TESTOBJS ARCH_HEADERS ARMV6-OBJS TOOLS
 
 define RESET