Add an MBED_USED attribute to main()

This attribute, attached to a function, means that code must be emitted
for the function even if it appears that the function is not referenced.

This is the case for combined "-Wl,--wrap,main" and "-flto" flags.

Author: Filip Jagodzinski

mbed.h

@@ -113,4 +113,6 @@ using namespace mbed;
 using namespace std;
 #endif
 
+MBED_USED int main(void);
+
 #endif