zlib: Move conditional compilation inside of the gzip.c

This implements stub for the compression functions and includes the
gzip.c in the compilation target uncoditionally, keeping the WITH_ZLIB
conditional compilation only in the gzip.c

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 0cd749a533)

src/CMakeLists.txt

@@ -91,6 +91,7 @@ set(libssh_SRCS
   ecdh.c
   error.c
   getpass.c
+  gzip.c
   init.c
   kdf.c
   kex.c
@@ -252,13 +253,6 @@ if (WITH_GEX)
   )
 endif (WITH_GEX)
 
-if (WITH_ZLIB)
-  set(libssh_SRCS
-    ${libssh_SRCS}
-    gzip.c
-  )
-endif(WITH_ZLIB)
-
 if (WITH_GSSAPI AND GSSAPI_FOUND)
   set(libssh_SRCS
     ${libssh_SRCS}