Don't exclude libssh2 from MinGW

It reportedly works. It does not however work when cross-compiling on
Travis, so let's disable it there.

This fixes #2311.

.travis.yml

@@ -19,7 +19,7 @@ matrix:
  fast_finish: true
  include:
    - compiler: i586-mingw32msvc-gcc
-     env: OPTIONS="-DBUILD_CLAR=OFF -DWIN32=ON -DMINGW=ON"
+     env: OPTIONS="-DBUILD_CLAR=OFF -DWIN32=ON -DMINGW=ON -DUSE_SSH=OFF"
    - compiler: gcc
      env: COVERITY=1
  allow_failures:

CMakeLists.txt

@@ -195,7 +195,7 @@ ELSE()
 ENDIF()
 
 # Optional external dependency: libssh2
-IF (USE_SSH AND NOT MINGW)
+IF (USE_SSH)
 	FIND_PACKAGE(LIBSSH2)
 ENDIF()
 IF (LIBSSH2_FOUND)