Fix warnings

[hanaxars]

Fix following gcc warnings when SVG enabled:

rtextures.c: In function 'LoadImageSvg':
rtextures.c:374:52: warning: pointer targets in passing argument 1 of 'nsvgParse' differ in signedness [-Wpointer-sign]
  374 |             struct NSVGimage *svgImage = nsvgParse(fileData, "px", 96.0f);
      |                                                    ^~~~~~~~
      |                                                    |
      |                                                    unsigned char *
In file included from rtextures.c:230:
external/nanosvg.h:2952:28: note: expected 'char *' but argument is of type 'unsigned char *'
 2952 | NSVGimage* nsvgParse(char* input, const char* units, float dpi)
      |                      ~~~~~~^~~~~
rtextures.c:407:43: warning: comparison of distinct pointer types lacks a cast [-Wcompare-distinct-pointer-types]
  407 |         if (isSvgStringValid && (fileData != fileNameOrString)) UnloadFileData(fileData);
      |                                           ^~
rtextures.c: In function 'LoadImageFromMemory':
rtextures.c:614:52: warning: passing argument 1 of 'nsvgParse' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  614 |             struct NSVGimage *svgImage = nsvgParse(fileData, "px", 96.0f);
      |                                                    ^~~~~~~~
external/nanosvg.h:2952:28: note: expected 'char *' but argument is of type 'const unsigned char *'
 2952 | NSVGimage* nsvgParse(char* input, const char* units, float dpi)
      |                      ~~~~~~^~~~~

[raysan5]

@hanaxars Thanks for the review!