perf: fix disabling BUILD_BPF_SKEL

The makefile uses ifdef to check, so the variable needs to be empty

Signed-off-by: Felix Fietkau <[email protected]>

package/devel/perf/Makefile

@@ -59,7 +59,7 @@ MAKE_FLAGS = \
 	NO_LIBCAP=1 \
 	NO_JEVENTS=1 \
 	NO_SHELLCHECK=1 \
-	BUILD_BPF_SKEL=0 \
+	BUILD_BPF_SKEL= \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
 	CC="$(TARGET_CC)" \
 	LD="$(TARGET_CROSS)ld" \