meson: Declare a find_program() override for mapperx

If phosphor-objmgr is used as a subproject, allow discovery of its
mapperx program for use in the parent project.

Change-Id: I1cb448500cab015b8b309979ab26f7e51f03e8ea
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 463c43a..4bd4c3f 100644
--- a/meson.build
+++ b/meson.build
@@ -88,7 +88,7 @@
     dependencies: [ dependency('libsystemd') ],
     install: true)
 
-executable(
+mapperx = executable(
     'mapperx',
     [
         'src/main.cpp',
@@ -110,6 +110,7 @@
     install_dir: join_paths(
         get_option('prefix'), get_option('libexecdir'), meson.project_name())
 )
+meson.override_find_program('mapperx', mapperx)
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
     'systemdsystemunitdir'