meson: Override dependencies to improve usage as a subproject

With this change, sysprof can be consumed as a subproject without
making any changes to the build files of a project. All you need to do
is provide a wrap file with a `[provide]` section:

https://mesonbuild.com/Wrap-dependency-system-manual.html#provide-section

This is also necessary because otherwise projects need to hard-code
the subproject name, which might be `sysprof` when using `wrap-git` or
`sysprof-3.46.0` when using `wrap-file` (to build from a release
tarball). This can cause conflicts between different subprojects that
consume sysprof differently.

Other projects like glib, cairo, pango, etc already do this.

src/libsysprof/meson.build

@@ -178,6 +178,7 @@ libsysprof_dep = declare_dependency(
          dependencies: libsysprof_deps,
   include_directories: [include_directories('.'), libsysprof_capture_include_dirs],
 )
+meson.override_dependency('sysprof-@0@'.format(libsysprof_api_version), libsysprof_dep)
 
 pkgconfig.generate(
                libsysprof,