Add L508-X6/T108 define.

Change-Id: I6a61386bfbc593919334cd8472a85134e4fd8919
diff --git a/mbtk/mbtk_lib/src/mbtk_gpio.c b/mbtk/mbtk_lib/src/mbtk_gpio.c
index 26c9f24..a5ec0e9 100755
--- a/mbtk/mbtk_lib/src/mbtk_gpio.c
+++ b/mbtk/mbtk_lib/src/mbtk_gpio.c
@@ -586,6 +586,20 @@
     return -1;
 }
 
+#elif defined(MBTK_PROJECT_L508_X6)
+
+int mbtk_at_gpio(void* arg)
+{
+    return -1;
+}
+
+#elif defined(MBTK_PROJECT_T108)
+
+int mbtk_at_gpio(void* arg)
+{
+    return -1;
+}
+
 #else
 
 int mbtk_at_gpio(void* arg)