perf: fix build with kernel 4.9

The musl workaround isn't required anymore and causes build errors with
kernel 4.9 when applied.

Signed-off-by: Mathias Kresin <dev@kresin.me>

package/devel/perf/Makefile

@@ -61,9 +61,11 @@ MAKE_FLAGS = \
 	WERROR=0 \
 	prefix=/usr
 
+ifdef CONFIG_LINUX_4_4
 ifdef CONFIG_USE_MUSL
  MAKE_FLAGS += EXTRA_CFLAGS="-I$(CURDIR)/musl-include -include $(CURDIR)/musl-compat.h -D__UCLIBC__"
 endif
+endif
 
 define Build/Compile
 	+$(MAKE_FLAGS) $(MAKE) $(PKG_JOBS) \