gettext: Fix ptest failure

msgmerge-29 is a script without execute permissions but executed as
./msgmerge-29 which results in this test failing.

It is only one test which fails on gettext/qemux86-64

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>