retarget: Added workaround for conflicting definitions of error_t

The errno.h header file defines the type error_t, unfortunately this
is a common type name that may be defined in user code. For at least GCC
we can work around this by telling errno that the error_t is already
defined.

platform/mbed_retarget.h

@@ -26,7 +26,18 @@
 #endif //__cplusplus
 #include <stdint.h>
 #include <stddef.h>
+
+/* Include logic for errno so we can get errno defined but not bring in error_t,
+ * including errno here prevents an include later, which would redefine our
+ * error codes
+ */
+#ifndef __error_t_defined
+#define __error_t_defined 1
 #include <errno.h>
+#undef __error_t_defined
+#else
+#include <errno.h>
+#endif
 
 /* 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