meson: Declare have_virtfs_proxy_helper in main meson.build

have_virtfs_proxy_helper is used from docs/meson.build, and can be
not declared when including it before fsdev/meson.build. This fixes:

  ../docs/meson.build:54:2: ERROR: Unknown variable "have_virtfs_proxy_helper".

Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: philmd

fsdev/meson.build

@@ -8,7 +8,6 @@ fsdev_ss.add(when: ['CONFIG_FSDEV_9P'], if_true: files(
 ), if_false: files('qemu-fsdev-dummy.c'))
 softmmu_ss.add_all(when: 'CONFIG_LINUX', if_true: fsdev_ss)
 
-have_virtfs_proxy_helper = have_tools and libattr.found() and libcap_ng.found() and have_virtfs
 if have_virtfs_proxy_helper
   executable('virtfs-proxy-helper',
              files('virtfs-proxy-helper.c', '9p-marshal.c', '9p-iov-marshal.c'),

meson.build

@@ -1034,6 +1034,8 @@ have_virtfs = (targetos == 'linux' and
     libattr.found() and
     libcap_ng.found())
 
+have_virtfs_proxy_helper = have_virtfs and have_tools
+
 if get_option('virtfs').enabled()
   if not have_virtfs
     if targetos != 'linux'