Avoid conflict with swap16/swap32 defines from endian.h

Index: src/freexl.c
--- src/freexl.c.orig
+++ src/freexl.c
@@ -108,6 +108,7 @@ lround (double num)
 }
 #endif
 
+#undef swap16
 static void
 swap16 (biff_word16 * word)
 {
@@ -117,6 +118,7 @@ swap16 (biff_word16 * word)
     word->bytes[1] = save;
 }
 
+#undef swap32
 static void
 swap32 (biff_word32 * word)
 {