compat: tfind: fix prototype

Use prototype from tsearch(3), fixes the following warning when
targetin Android arm API 17.

../../src/misc/variables.c:154:32: warning: passing 'void **' to
parameter of type 'const void **' discards qualifiers in nested
pointer types [-Wincompatible-pointer-types-discards-qualifiers]

    pp_var = tfind( &psz_name, &priv->var_root, varcmp );
                               ^~~~~~~~~~~~~~~

compat/tfind.c

@@ -132,7 +132,7 @@ tdestroy(vrootp, freefct)
 void *
 tfind(vkey, vrootp, compar)
 	const void *vkey;		/* key to be found */
-	const void **vrootp;		/* address of the tree root */
+	void * const *vrootp;		/* address of the tree root */
 	int (*compar) (const void *, const void *);
 {
 	node_t * const *rootp = (node_t * const*)vrootp;

include/vlc_fixups.h

@@ -497,7 +497,7 @@ typedef enum {
 } VISIT;
 
 void *tsearch( const void *key, void **rootp, int(*cmp)(const void *, const void *) );
-void *tfind( const void *key, const void **rootp, int(*cmp)(const void *, const void *) );
+void *tfind( const void *key, void * const *rootp, int(*cmp)(const void *, const void *) );
 void *tdelete( const void *key, void **rootp, int(*cmp)(const void *, const void *) );
 void twalk( const void *root, void(*action)(const void *nodep, VISIT which, int depth) );
 void *lfind( const void *key, const void *base, size_t *nmemb,