Common/Hash: Namespace code under the Common namespace

Brings more common code under the Common namespace.

Source/Core/Common/Hash.h

@@ -8,8 +8,11 @@
 
 #include "Common/CommonTypes.h"
 
+namespace Common
+{
 u32 HashFletcher(const u8* data_u8, size_t length);  // FAST. Length & 1 == 0.
 u32 HashAdler32(const u8* data, size_t len);         // Fairly accurate, slightly slower
 u32 HashEctor(const u8* ptr, int length);            // JUNK. DO NOT USE FOR NEW THINGS
 u64 GetHash64(const u8* src, u32 len, u32 samples);
 void SetHash64Function();
+}  // namespace Common