Keyboardio: Change from static to extern

We want the `Keyboardio` object to be a singleton. If it is static, then
each library that gets compiled separately, and uses it in one way or
the other, will have a copy of it.

Making it extern, we'll only have one copy, as it should be.

I don't think there were any bugs caused by it being static, but it was
certainly a tiny bit of wasted code and memory.

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

src/KeyboardioFirmware.cpp

@@ -53,3 +53,5 @@ Keyboardio_::use(KeyboardioPlugin *plugin, ...) {
     };
     va_end(ap);
 }
+
+Keyboardio_ Keyboardio;

src/KeyboardioFirmware.h

@@ -43,4 +43,4 @@ class Keyboardio_ {
     void use(KeyboardioPlugin *plugin, ...) __attribute__((sentinel));
 };
 
-static Keyboardio_ Keyboardio;
+extern Keyboardio_ Keyboardio;