Reapply "osdep/meson: add libplacebo include dir to swift compile"

It's not actually related to libplacebo wrap stuff and the swift compile
command needs this to get the right libplacebo include path.

This reverts commit b9d392ecd9.

osdep/meson.build

@@ -25,7 +25,9 @@ swift_flags += extra_flags
 swift_compile = [swift_prog, swift_flags, '-module-name', 'macOS_swift',
                  '-emit-module-path', '@OUTPUT0@', '-import-objc-header', bridge,
                  '-emit-objc-header-path', '@OUTPUT1@', '-o', '@OUTPUT2@',
-                 '@INPUT@', '-I.', '-I' + source_root]
+                 '@INPUT@', '-I.', '-I' + source_root,
+                 '-I' + libplacebo.get_variable('includedir',
+                            default_value: source_root / 'subprojects' / 'libplacebo' / 'src' / 'include')]
 
 swift_targets = custom_target('swift_targets',
     input: swift_sources,