UI: Quiet warning when setting panel type description from C

No user visible changes expected.

Assigning a string literal to `PanelType.description` would give a
warning, because ISO C requires these to be `const char *`. Wasn't used
so far, but will be with #104831.

source/blender/blenkernel/BKE_screen.h

@@ -255,7 +255,7 @@ typedef struct PanelType {
 
   char idname[BKE_ST_MAXNAME]; /* unique name */
   char label[BKE_ST_MAXNAME];  /* for panel header */
-  char *description;           /* for panel tooltip */
+  const char *description;     /* for panel tooltip */
   char translation_context[BKE_ST_MAXNAME];
   char context[BKE_ST_MAXNAME];   /* for buttons window */
   char category[BKE_ST_MAXNAME];  /* for category tabs */