retarget: Added include errno.h

Unfortunately, some toolchains don't define the same errno codes that
are used fairly consistently on Linux based platforms, which means they
also don't match the errno codes used in the retarget layer.

If a user includes errno.h after mbed.h, the errno codes can be
redefined incorrectly.

Adding an include of errno.h in mbed.h forces the order to be fixed.

Author: geky

platform/mbed_retarget.h

@@ -26,6 +26,7 @@
 #endif //__cplusplus
 #include <stdint.h>
 #include <stddef.h>
+#include <errno.h>
 
 /* We can get the following standard types from sys/types for gcc, but we
  * need to define the types ourselves for the other compilers that normally