gcc: fix build error with macOS + Xcode 9

Signed-off-by: Felix Fietkau <nbd@nbd.name>

toolchain/gcc/patches/5.4.0/030-macos-header-fix-backport.patch

@@ -0,0 +1,10 @@
+--- a/gcc/system.h
++++ b/gcc/system.h
+@@ -216,6 +216,7 @@ extern int errno;
+ 
+ #ifdef __cplusplus
+ # include <algorithm>
++# include <map>
+ # include <cstring>
+ # include <utility>
+ #endif