Include <rpmpgp.h> for the rpmFreeCrypto function, to avoid an
implicit function declaration.

Submitted upstream:

  <https://github.com/linux-application-whitelisting/fapolicyd/pull/212>

diff --git a/src/library/rpm-backend.c b/src/library/rpm-backend.c
index e5c5f3318285fd4b..6122ef1bf6623522 100644
--- a/src/library/rpm-backend.c
+++ b/src/library/rpm-backend.c
@@ -30,6 +30,7 @@
 #include <rpm/rpmmacro.h>
 #include <rpm/rpmlog.h>
 #include <rpm/rpmdb.h>
+#include <rpm/rpmpgp.h>
 #include <fnmatch.h>
 
 #include <uthash.h>