package/conmon: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since the addition
of the package in commit 06f50ff1bf066feb6cf6ed512113773f87c884fb:

src/conmon.c:5:2: error: #error conmon.c requires C99 or later
 #error conmon.c requires C99 or later
  ^

Fixes: 06f50ff1bf066feb6cf6ed512113773f87c884fb
 - http://autobuild.buildroot.org/results/b573aceefde04435ea13dfd2a48f9c2372bde4d7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/conmon/conmon.mk

@@ -25,7 +25,8 @@ define CONMON_CONFIGURE_CMDS
 endef
 
 define CONMON_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) CC="$(TARGET_CC)" CFLAGS="$(TARGET_CFLAGS)" \
+	$(TARGET_MAKE_ENV) $(MAKE) CC="$(TARGET_CC)" \
+		CFLAGS="$(TARGET_CFLAGS) -std=c99" \
 		LDFLAGS="$(TARGET_LDFLAGS)" -C $(@D) bin/conmon
 endef