Fix typecasting warnings

[Sharpie]

These have been around for ever and should be cleaned up:

tikzDevice.c:286:10: warning: passing 'const char *' to parameter of type 'char *' discards qualifiers [-Wincompatible-pointer-types]
  strcpy(tikzInfo->documentDeclaration, documentDeclaration);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/secure/_string.h:88:30: note: expanded from macro 'strcpy'
   ? __builtin___strcpy_chk (dest, src, __darwin_obsz (dest))           \

tikzDevice.c:580:8: warning: passing 'const char *' to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types]
  free(tikzInfo->packages);
       ^~~~~~~~~~~~~~~~~~
/usr/include/stdlib.h:160:18: note: passing argument to parameter here
void     free(void *);