package/janet: add -std=c99 to CFLAGS

As upstream says Janet should be compiled with -std=c99
so pass this in order to fix the failures below.

Fixes:
 http://autobuild.buildroot.net/results/192/1922ec115b9dc9e879760a6868969d9677093510/build-end.log
 ...
 http://autobuild.buildroot.net/results/355/355e0992338a8d132050517f83a3884606b00529/build-end.log

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/janet/janet.mk

@@ -8,8 +8,8 @@ JANET_VERSION = 1.15.0
 JANET_SITE = $(call github,janet-lang,janet,v$(JANET_VERSION))
 JANET_LICENSE = MIT
 JANET_LICENSE_FILES = LICENSE
-
 JANET_INSTALL_STAGING = YES
+JANET_CFLAGS = $(TARGET_CFLAGS) -std=c99
 
 ifeq ($(BR2_STATIC_LIBS),y)
 JANET_CONF_OPTS += -Ddynamic_modules=false