Fix compilation with GLib older than 2.10.

--- gwyddion-2.8/libprocess/simplefft.c.orig	2007-07-10 21:06:40.000000000 +0200
+++ gwyddion-2.8/libprocess/simplefft.c	2007-09-13 11:24:07.000000000 +0200
@@ -168,7 +168,7 @@
 #else
 #define _gwy_scratch_buffer_free_backend(b, l) \
     g_free(b)
-#define _gwy_scratch_buffer_alloc_backend(b, l) \
+#define _gwy_scratch_buffer_alloc_backend(l) \
     g_malloc((l)*sizeof(gdouble) + sizeof(_GwyScratchBufferHead))
 #endif