Make pp_help a constant pointer to constant characters, moves it partially

in .rodata (the actual string) and partially in .data.relro (the pointer),
but doesn't change ABI.
Patch by Diego 'Flameeyes' Pettenò  flameeyes at gmail com

Originally committed as revision 11390 to svn://svn.ffmpeg.org/ffmpeg/trunk

libpostproc/postprocess.c

@@ -697,7 +697,7 @@ static inline void postProcess(uint8_t src[], int srcStride, uint8_t dst[], int
 
 /* -pp Command line Help
 */
-char *pp_help=
+const char *const pp_help=
 "Available postprocessing filters:\n"
 "Filters                        Options\n"
 "short  long name       short   long option     Description\n"

libpostproc/postprocess.h

@@ -42,7 +42,7 @@
 typedef void pp_context_t;
 typedef void pp_mode_t;
 
-extern char *pp_help; ///< a simple help text
+extern const char *const pp_help; ///< a simple help text
 
 void  pp_postprocess(uint8_t * src[3], int srcStride[3],
                  uint8_t * dst[3], int dstStride[3],