testsuite: libgomp: fix dg-do run typo

'dg-run' is not a valid dejagnu directive, 'dg-do run' is needed here
for the test to be executed.

That said, it actually seems to be executed for me anyway, presumably
a default in the directory, but let's fix it to be consistent with
other uses in the tree and in that test directory even.

libgomp/ChangeLog:
	* testsuite/libgomp.c++/declare-target-indirect-1.C: Fix 'dg-run' typo.