Fix FTBFS on Precise

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4621f92..51e85fa 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -698,6 +698,7 @@
   set( HAVE_CRYPTSETUP 1 )
   if ( HAVE_CRYPTSETUP )
     check_c_source_compiles("
+      #include <stddef.h>
       #include <libcryptsetup.h>
       int main(int argc, char *argv[]) { crypt_keyslot_info status = CRYPT_SLOT_INVALID; return 0; } "
       HAVE_NEW_CRYPTSETUP )