Update remote and version of submodule rpmalloc (#5696)

This PR updates the remote URL and content of rpmalloc.

Fixes #4752
Fixes #4806 (assumably)
Helps #5694

You must now run
```
git submodule sync --recursive
```
once to reflect this change.

.gitmodules

@@ -3,7 +3,7 @@
 	url = https://github.com/Lukas-W/qt5-x11embed.git
 [submodule "src/3rdparty/rpmalloc/rpmalloc"]
 	path = src/3rdparty/rpmalloc/rpmalloc
-	url = https://github.com/rampantpixels/rpmalloc.git
+	url = https://github.com/mjansson/rpmalloc.git
 [submodule "plugins/zynaddsubfx/zynaddsubfx"]
 	path = plugins/zynaddsubfx/zynaddsubfx
 	url = https://github.com/lmms/zynaddsubfx.git

src/3rdparty/rpmalloc/CMakeLists.txt

@@ -21,6 +21,12 @@ if (NOT LMMS_BUILD_WIN32)
 	)
 endif()
 
+if(MINGW)
+	target_compile_definitions(rpmalloc
+		PRIVATE -D_WIN32_WINNT=0x600
+	)
+endif()
+
 if (CMAKE_BUILD_TYPE STREQUAL "Debug")
 	# rpmalloc uses GCC builtin "__builtin_umull_overflow" with ENABLE_VALIDATE_ARGS,
 	# which is only available starting with GCC 5