xx_hasher: disable -Warray-bounds

In release mode gcc9 has a false positive warning about out of bound
access in xxhash implementation:

./xxHash/xxhash.c:799:27: error: array subscript -3 is outside array bounds of ‘long unsigned int [1]’ [-Werror=array-bounds]

This is solved by disabling -Warray-bounds in the xxhash code.

xx_hasher.hh

@@ -24,7 +24,11 @@
 #include "bytes.hh"
 #include "utils/serialization.hh"
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Warray-bounds"
 #include <xxHash/xxhash.h>
+#pragma GCC diagnostic pop
+
 #include <array>
 
 class xx_hasher {