Allow c99 when building ZFS in the kernel tree

Commit 4a5d7f8 enabled building c99 out of the kernel tree.
However, when building as part of the kernel different Makefiles
are used and -std=gnu99 must additionially be added there.

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#5919

Author: behlendorf

copy-builtin

@@ -59,8 +59,11 @@ EOF
 
 {
 	cat <<-"EOF"
-	ZFS_MODULE_CFLAGS  = -I$(srctree)/include/zfs -I$(srctree)/include/spl 
-	ZFS_MODULE_CFLAGS += -include $(srctree)/spl_config.h -include $(srctree)/zfs_config.h
+	ZFS_MODULE_CFLAGS  = -I$(srctree)/include/zfs
+	ZFS_MODULE_CFLAGS += -I$(srctree)/include/spl
+	ZFS_MODULE_CFLAGS += -include $(srctree)/spl_config.h
+	ZFS_MODULE_CFLAGS += -include $(srctree)/zfs_config.h
+	ZFS_MODULE_CLFAGS += -std=gnu99
 	export ZFS_MODULE_CFLAGS
 
 	obj-$(CONFIG_ZFS) :=