osmo-mgw.spec.in: Fix dependency to libosmoabis

Confusingly, in Debian the package is called libosmo-abis, but in
the CentOS/RPM it's called libosmoabis.  Fixes a bug introduced
in I45717bda3ef7eba1ef59b993cc8a69bf2f92a29f

Change-Id: I97dfec72a99295148e84e60c1e5037381f736c03
diff --git a/contrib/osmo-mgw.spec.in b/contrib/osmo-mgw.spec.in
index a34a715..0f96ed0 100644
--- a/contrib/osmo-mgw.spec.in
+++ b/contrib/osmo-mgw.spec.in
@@ -35,7 +35,7 @@
 BuildRequires:  pkgconfig(libosmogsm) >= 1.0.0
 BuildRequires:  pkgconfig(libosmovty) >= 1.0.0
 BuildRequires:  pkgconfig(libosmocoding) >= 1.0.0
-BuildRequires:  pkgconfig(libosmo-abis)
+BuildRequires:  pkgconfig(libosmoabis)
 %{?systemd_requires}
 
 %description