Remove undef f from SysCall.h

When SdFat.h is included it was undefining the F() (flash string) macro.
This causes all strings to go into RAM, as seen in this issue:
esp8266/Arduino#6767

Remove this undef.  The F() is only used in a few print statements which
are F()-safe on the 8266 now, and in the SD.h and SDFS.h libs the
function that actually calls print(F(xxxx)) is never used.

Author: earlephilhower

src/SysCall.h

@@ -42,13 +42,7 @@ namespace sdfat {
 
 //------------------------------------------------------------------------------
 #ifdef ESP8266
-// undefine F macro if ESP8266.
-#undef F
-#endif  // ESP8266
-//------------------------------------------------------------------------------
-#ifndef F
-/** Define macro for strings stored in flash. */
-#define F(str) (str)
+#include <pgmspace.h>
 #endif  // F
 //------------------------------------------------------------------------------
 /** \return the time in milliseconds. */