package/tree: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since bump to
version 2.0.2 in commit 11a3989dbf7ac2535fac04b24ab3d959da3686b3:

color.c: In function 'parse_dir_colors':
color.c:103:11: error: redeclaration of 'i' with no linkage
   for(int i=0; i < DOT_EXTENSION; i++) color_code[i] = NULL;
           ^

Fixes:
 - http://autobuild.buildroot.org/results/e05595696727622eaf517944d4ff4414baf66bb2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/tree/tree.mk

@@ -11,7 +11,8 @@ TREE_LICENSE = GPL-2.0+
 TREE_LICENSE_FILES = LICENSE
 
 define TREE_BUILD_CMDS
-	$(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D)
+	$(MAKE) $(TARGET_CONFIGURE_OPTS) CFLAGS="$(TARGET_CFLAGS) -std=c99" \
+		-C $(@D)
 endef
 
 define TREE_INSTALL_TARGET_CMDS