Add static lib to -dev .deb package

According to https://www.debian.org/doc/debian-policy/ch-sharedlibs.html
and http://packaging.ubuntu.com/html/libraries.html that's where it
belongs.
diff --git a/debian/libosmocore-dev.install b/debian/libosmocore-dev.install
index 7a62c6f..7c5a789 100644
--- a/debian/libosmocore-dev.install
+++ b/debian/libosmocore-dev.install
@@ -1,4 +1,5 @@
 usr/include/*
+usr/lib/*/lib*.a
 usr/lib/*/lib*.so
 usr/lib/*/lib*.la
 usr/lib/*/pkgconfig/*