Fix includes for musl libc

[thyrc]

MemoryWatcher.cpp is missing the cstring include for strncpy.

On some platforms the missing header is included implicitly, but on musl we'll have to include cstring to build dolphin successfully.

./Source/Core/Core/MemoryWatcher.cpp: In member function 'bool MemoryWatcher::OpenSocket(const string&)':
./Source/Core/Core/MemoryWatcher.cpp:63:3: error: 'strncpy' was not declared in this scope
   63 |   strncpy(m_addr.sun_path, path.c_str(), sizeof(m_addr.sun_path) - 1);
      |   ^~~~~~~
./Source/Core/Core/MemoryWatcher.cpp:14:1: note: 'strncpy' is defined in header '<cstring>'; did you forget to '#include <cstring '?
   13 | #include "Core/PowerPC/MMU.h"
  +++ |+#include <cstring>
   14 |

[JosJuice]

LGTM if you amend the commit with a lint fix.

[JosJuice]

The code looks good now, but we prefer that little changes like lint fixes are amended to the original commit instead of being added in a second commit.