Define USER_BTN for BLACK PILL F411CE

Thanks @BennehBoy

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

variants/Generic_F411Cx/variant.h

@@ -81,7 +81,10 @@ extern "C" {
 #define LED_GREEN               LED_BUILTIN
 
 // On-board user button
-#if !defined(USER_BTN) && !defined(ARDUINO_BLACKPILL_F411CE)
+#ifdef ARDUINO_BLACKPILL_F411CE
+#define USER_BTN                PA0
+#endif
+#if !defined(USER_BTN)
 #define USER_BTN                PC13
 #endif