debian: Adapt package name to lib's current major version

Current LIBVERSION is 1:1:1 (current:revision:age), so major=current-age
makes it be major=0: libgtpnl0.

It can bee seen in current osmocom debian repos that libgtpnl1 package
contains libgtpnl.so.0.

Let's set package name accordingly.

Change-Id: I7d2fa50b6a17a598467f555558660ef2396c1744

debian/control

@@ -16,7 +16,7 @@ Vcs-Git: git://git.osmocom.org/libgtpnl.git
 Vcs-Browser: http://git.osmocom.org/gitweb?p=libgtpnl.git;a=summary
 Homepage: https://projects.osmocom.org/projects/openggsn
 
-Package: libgtpnl1
+Package: libgtpnl0
 Section: libs
 Architecture: any
 Multi-Arch: same
@@ -29,7 +29,7 @@ Architecture: any
 Multi-Arch: same
 Section: libdevel
 Depends: libmnl-dev,
-         libgtpnl1 (= ${binary:Version}),
+         libgtpnl0 (= ${binary:Version}),
          ${misc:Depends}
 Description: Development headers for Linux kernel GTP-U netlink library
  The header files provided by this package may be used to develop
@@ -40,6 +40,6 @@ Architecture: any
 Multi-Arch: same
 Section: debug
 Priority: extra
-Depends: libgtpnl1 (= ${binary:Version}),
+Depends: libgtpnl0 (= ${binary:Version}),
          ${misc:Depends}
 Description: Debug symbols for Linux kernel GTP-U netlink library