Fix build issues when RTT is enabled

Fixes raspberrypi/pico-examples#565

Author: peterharperuk

src/rp2_common/pico_stdio_rtt/SEGGER/RTT/SEGGER_RTT.c

@@ -78,6 +78,11 @@ Additional information:
 
 #include <string.h>                 // for memcpy
 
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wcast-qual"
+#pragma GCC diagnostic ignored "-Wcast-align"
+#endif
+
 /*********************************************************************
 *
 *       Configuration, default values

src/rp2_common/pico_stdio_rtt/stdio_rtt.c

@@ -26,11 +26,11 @@ void stdio_rtt_deinit(void) {
 }
 
 static void stdio_rtt_out_chars(const char *buf, int length) {
-    SEGGER_RTT_Write(0, buf, length);
+    SEGGER_RTT_Write(0, buf, (unsigned)length);
 }
 
 static int stdio_rtt_in_chars(char *buf, int length) {
-    return SEGGER_RTT_Read(0, buf, length);
+    return (int)SEGGER_RTT_Read(0, buf, (unsigned)length);
 }
 
 stdio_driver_t stdio_rtt = {