build: add missing gcc versions to conf

amd64/include/cflags.json

@@ -21,6 +21,9 @@
 			],
 			"/usr/bin/gcc": [
 				"-Wno-frame-address"
+			],
+			"/usr/bin/gcc-6": [
+				"-Wno-frame-address"
 			]
 		},
 		"Cflags": [

amd64/include/klib.json

@@ -6,6 +6,9 @@
 			],
 			"/usr/bin/gcc": [
 				"-Wno-frame-address"
+			],
+			"/usr/bin/gcc-6": [
+				"-Wno-frame-address"
 			]
 		},
 		"Cflags": [

sys/src/9/amd64/core.json

@@ -18,6 +18,9 @@
 			],
 			"/usr/bin/gcc": [
 				"-Wno-frame-address"
+			],
+			"/usr/bin/gcc-6": [
+				"-Wno-frame-address"
 			]
 		},
 		"Cflags": [