Common: Add MemArena.h/cpp to the Common namespace

Brings more common code under the Common namespace.

Source/Core/Common/MemArena.cpp

@@ -28,6 +28,8 @@
 #endif
 #endif
 
+namespace Common
+{
 #ifdef ANDROID
 #define ASHMEM_DEVICE "/dev/ashmem"
 
@@ -154,3 +156,5 @@ u8* MemArena::FindMemoryBase()
   return static_cast<u8*>(base);
 #endif
 }
+
+}  // namespace Common

Source/Core/Common/MemArena.h

@@ -12,11 +12,12 @@
 
 #include "Common/CommonTypes.h"
 
+namespace Common
+{
 // This class lets you create a block of anonymous RAM, and then arbitrarily map views into it.
 // Multiple views can mirror the same section of the block, which makes it very convenient for
 // emulating
 // memory mirrors.
-
 class MemArena
 {
 public:
@@ -35,3 +36,5 @@ private:
   int fd;
 #endif
 };
+
+}  // namespace Common

Source/Core/Core/HW/Memmap.cpp

@@ -44,7 +44,7 @@ u8* physical_base = nullptr;
 u8* logical_base = nullptr;
 
 // The MemArena class
-static MemArena g_arena;
+static Common::MemArena g_arena;
 // ==============
 
 // STATE_TO_SAVE
@@ -190,7 +190,7 @@ void Init()
     mem_size += region.size;
   }
   g_arena.GrabSHMSegment(mem_size);
-  physical_base = MemArena::FindMemoryBase();
+  physical_base = Common::MemArena::FindMemoryBase();
 
   for (PhysicalMemoryRegion& region : physical_regions)
   {