preload: hoist preloads into src/ directory

They will get used from libsysprof-profile, but it's nice to have them
a directory up going forward.

src/libsysprof/meson.build

@@ -196,5 +196,3 @@ pkgconfig.generate(
 
 install_headers(libsysprof_public_headers, subdir: sysprof_header_subdir)
 endif
-
-subdir('preload')

src/meson.build

@@ -64,6 +64,7 @@ endif
 
 if need_libsysprof_profile
   subdir('libsysprof-profile')
+  subdir('preload')
 endif
 
 if get_option('sysprofd') == 'bundled'