file: don't force _GNU_SOURCE in CFLAGS

Breaks the build with BR2_CONFIG_CACHE, and isn't needed anyway, as
the configure script figures out to add _GNU_SOURCE to config.h
anyway.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/file/file.mk

@@ -6,7 +6,6 @@
 
 FILE_VERSION = 5.04
 FILE_SITE = ftp://ftp.astron.com/pub/file/
-FILE_CONF_ENV = CFLAGS="$(TARGET_CFLAGS) -D_GNU_SOURCE"
 FILE_DEPENDENCIES = host-file zlib
 HOST_FILE_DEPENDENCIES = host-zlib