msvcrt: fix a compiler warning on PowerPC

[Austin English]

Tested on x86/ppc.

-- 
-Austin
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diff --git a/dlls/msvcrt/file.c b/dlls/msvcrt/file.c
index 6671f2f..6b0ac42 100644
--- a/dlls/msvcrt/file.c
+++ b/dlls/msvcrt/file.c
@@ -2405,7 +2405,7 @@ char * CDECL MSVCRT_fgets(char *s, int size, MSVCRT_FILE* file)
  */
 MSVCRT_wint_t CDECL MSVCRT_fgetwc(MSVCRT_FILE* file)
 {
-  char c;
+  signed char c;
 
   if (!(MSVCRT_fdesc[file->_file].wxflag & WX_TEXT))
     {