wmbattery: Avoid redefinition of apm_read/apm_exists in *BSD.

The two functions are defined if the BSD-specific apm headers are found and
if libapm is not found.  Both of these conditions will be true on BSD
systems, so we use #elif instead of the separate #if statements.

wmbattery/wmbattery.c

@@ -147,8 +147,7 @@ int apm_exists(void)
 		return 0;
 	return apm_read(&i);
 }
-#endif
-#if !defined(HAVE_LIBAPM)
+#elif !defined(HAVE_LIBAPM)
 int apm_read(apm_info *i)
 {
 	return -1;