meson: refactor generators

Using 'run_command' is not the appropriate way to generate code
since it only runs when meson itself runs and doesn't expose to
meson the correct dependency information.  'custom_target' is
the more appropriate way to generate code, so switch to using it.

This was noticed because when trying to build as a subproject, the
depending repository failed with:
```
subprojects/phosphor-host-ipmid/meson.build:229:0: ERROR: Sandbox
violation: Tried to grab file sensor-gen.cpp outside current
(sub)project.
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I555c6e1a7bfaabae63d757042ddc1c5a548295aa
diff --git a/meson.build b/meson.build
index 3ecd444..402c3a7 100644
--- a/meson.build
+++ b/meson.build
@@ -100,6 +100,8 @@
     nlohmann_json_dep = dependency('nlohmann-json')
 endif
 
+generated_src = []
+
 # Subfolders
 subdir('libipmid')
 subdir('libipmid-host')
@@ -181,12 +183,6 @@
   mapper,
 ]
 
-generated_src = [
-  meson.project_build_root() + '/sensor-gen.cpp',
-  meson.project_build_root() + '/inventory-sensor-gen.cpp',
-  meson.project_build_root() + '/fru-read-gen.cpp',
-]
-
 transportoem_src = []
 if not get_option('transport-oem').disabled()
   transportoem_src = ['transporthandler_oem.cpp']