Added fprintf fputc and _StdErr definitions

PX4 uses fprintf in numerous places but it is not supported by qurt.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/qurt/px4_layer/px4_qurt_impl.cpp

@@ -166,3 +166,15 @@ int write(int a, char const *b, int c)
 {
 	return -1;
 }
+
+int fprintf(FILE *stream, const char *format, ...)
+{
+	return 0;
+}
+
+int fputc(int c, FILE *stream)
+{
+	return c;
+}
+
+FILE _Stderr;