allow a patch that only contains hook functions

[jpoimboe]

The following patch isn't currently allowed:

Index: src/fs/aio.c
===================================================================
--- src.orig/fs/aio.c
+++ src/fs/aio.c
@@ -1642,3 +1642,15 @@ SYSCALL_DEFINE5(io_getevents, aio_contex
    }
    return ret;
 }
+
+void kpatch_load_aio_max_nr(void)
+{
+   aio_max_nr = 0x40000;
+}
+void kpatch_unload_aio_max_nr(void)
+{
+   aio_max_nr = 0x10000;
+}
+#include "kpatch-macros.h"
+KPATCH_LOAD_HOOK(kpatch_load_aio_max_nr);
+KPATCH_UNLOAD_HOOK(kpatch_unload_aio_max_nr);

$ kpatch-build/kpatch-build -t vmlinux ~/aio.patch 
Using cache at /home/jpoimboe/.kpatch/src
Testing patch file
checking file fs/aio.c
Building original kernel
Building patched kernel
Extracting new and modified ELF sections
aio.o: found hook: kpatch_unload_aio_max_nr
aio.o: found hook: kpatch_load_aio_max_nr
aio.o: no changed functions were found
ERROR: no functional changes found