Fix compile error in pgm_read_ptr

Acording to 
esp8266/Arduino#5733
esp8266/Arduino#5735

Author: tomec-martin

src/arduino/pgmspace.h

@@ -103,7 +103,7 @@ typedef const void* uint_farptr_t;
 #define pgm_read_word(addr) (*(const unsigned short *)(addr))
 #define pgm_read_dword(addr) (*(const unsigned long *)(addr))
 #define pgm_read_float(addr) (*(const float *)(addr))
-#define pgm_read_ptr(addr) (*(const void *)(addr))
+#define pgm_read_ptr(addr) (*(const void * const *)(addr))
 
 #define pgm_read_byte_near(addr) pgm_read_byte(addr)
 #define pgm_read_word_near(addr) pgm_read_word(addr)
@@ -119,4 +119,4 @@ typedef const void* uint_farptr_t;
 
 #define pgm_get_far_address(addr) (&(addr))
 
-#endif
+#endif