Extend the -Warray-bounds workaround in FixedString.h to gcc-5.

Summary: FixedString.h has a workaround for a gcc bug regarding erroneous -Werror=array-bounds errors. This commit extends the workaround to the gcc-5 series, which sadly is still broken. Life gets better with gcc-6.

Reviewed By: luciang

Differential Revision: D5210633

fbshipit-source-id: 4fcf080d18ddc573636a3e58b61a97167a8c06ce

folly/FixedString.h

@@ -120,7 +120,7 @@ enum class Cmp : int { LT = -1, EQ = 0, GT = 1 };
 
 // Rather annoyingly, GCC's -Warray-bounds warning issues false positives for
 // this code. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61971
-#if defined(__GNUC__) && !defined(__CLANG__) && __GNUC__ <= 4
+#if defined(__GNUC__) && !defined(__CLANG__) && __GNUC__ <= 5
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Warray-bounds"
 #endif