configure: fix $nm

Originally committed as revision 16104 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1267,7 +1267,7 @@ test -n "$cross_prefix" && enable cross_compile
 cc_default="${cross_prefix}${cc_default}"
 yasmexe="${cross_prefix}${yasmexe}"
 ar="${cross_prefix}${ar}"
-nm_default="${cross_prefix}${nm}"
+nm_default="${cross_prefix}${nm_default}"
 ranlib="${cross_prefix}${ranlib}"
 strip="${cross_prefix}${strip}"