gresource tests: fix VPATH builds emitting errors during dependency generation

gresource tests: fix VPATH builds emitting errors during dependency generation

Adding the --sourcedir option fixes these:
/path/to/src/gio/tests/test2.gresource.xml: Error on line 5 char 1: Failed to locate 'test1.txt' in current directory.
/path/to/src/gio/tests/test3.gresource.xml: Error on line 5 char 1: Failed to locate 'test1.txt' in current directory.
/path/to/src/gio/tests/test4.gresource.xml: Error on line 5 char 1: Failed to locate 'test1.txt' in current directory.
/path/to/src/gio/tests/test.gresource.xml: Error on line 5 char 1: Failed to locate 'test1.txt' in current directory.