scanner: Guard interface declarations

This allows to include client and server headers in the same file
fixing warnings like

In file included from ../subprojects/wlroots/include/wlr/types/wlr_layer_shell_v1.h:16,
                 from ../src/desktop.h:16,
                 from ../src/server.h:13,
                 from ../tests/testlib.c:8:
tests/59830eb@@footest@sta/wlr-layer-shell-unstable-v1-protocol.h:80:34: warning: redundant redeclaration of ‘zwlr_layer_shell_v1_interface’ [-Wredundant-decls]
   80 | extern const struct wl_interface zwlr_layer_shell_v1_interface;
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../tests/testlib.h:8,
                 from ../tests/testlib.c:7:
tests/59830eb@@footest@sta/wlr-layer-shell-unstable-v1-client-protocol.h:77:34: note: previous declaration of ‘zwlr_layer_shell_v1_interface’ was here
   77 | extern const struct wl_interface zwlr_layer_shell_v1_interface;
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../subprojects/wlroots/include/wlr/types/wlr_layer_shell_v1.h:16,
                 from ../src/desktop.h:16,
                 from ../src/server.h:13,
                 from ../tests/testlib.c:8:
tests/59830eb@@footest@sta/wlr-layer-shell-unstable-v1-protocol.h:106:34: warning: redundant redeclaration of ‘zwlr_layer_surface_v1_interface’ [-Wredundant-decls]
  106 | extern const struct wl_interface zwlr_layer_surface_v1_interface;
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../tests/testlib.h:8,
                 from ../tests/testlib.c:7:
tests/59830eb@@footest@sta/wlr-layer-shell-unstable-v1-client-protocol.h:103:34: note: previous declaration of ‘zwlr_layer_surface_v1_interface’ was here
  103 | extern const struct wl_interface zwlr_layer_surface_v1_interface;

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Closes: #158