MouseKeys: Make MouseKeys static

When we mark a symbol extern, but do not reference it anywhere else
directly, it will not be compiled in when using dot_a_linkage. For this
reason, make MouseKeys a static variable instead of extern.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

libraries/Keyboardio-MouseKeys/src/Keyboardio-MouseKeys.cpp

@@ -56,5 +56,3 @@ static bool handleMouseKeys(Key mappedKey, byte row, byte col, uint8_t keyState)
 MouseKeys_::MouseKeys_(void) {
     event_handler_hook_add (handleMouseKeys);
 }
-
-MouseKeys_ MouseKeys;

libraries/Keyboardio-MouseKeys/src/Keyboardio-MouseKeys.h

@@ -7,4 +7,4 @@ class MouseKeys_ {
     MouseKeys_ (void);
 };
 
-extern MouseKeys_ MouseKeys;
+static MouseKeys_ MouseKeys;