Add an errno.h include to freespace.c to fix bug 28974.

changes/bug28974

@@ -0,0 +1,3 @@
+  o Minor bugfixes (compilation):
+    - Fix compilation for Android by adding a missing header to
+      freespace.c.  Fixes bug 28974; bugfix on 0.3.5.1-alpha.

src/lib/fs/freespace.c

@@ -19,6 +19,7 @@
 #include <windows.h>
 #endif
 
+#include <errno.h>
 #include <string.h>
 
 /** Return the amount of free disk space we have permission to use, in