demote osinfo-db as libguestfs-tools-c suggest

it is not used anymore by the library, but only by one of the tools in
libguestfs-tools-c -- hence, demote it to suggest

libguestfs.spec

@@ -201,7 +201,6 @@ Requires:      yajl%{?_isa}
 
 # For core inspection API.
 Requires:      libdb-utils
-Requires:      osinfo-db
 
 # For core mount-local (FUSE) API.
 Requires:      fuse
@@ -477,6 +476,9 @@ Requires:      xz
 #Requires:     nbdkit, nbdkit-plugin-xz
 Requires:      curl
 
+# For virt-builder-repository:
+Suggests:      osinfo-db
+
 # Some Fedora, and all RHEL 7, use XFS:
 Recommends:    libguestfs-xfs