oeqa/selftest/cases/gcc.py: add libitm to default installed packages.

oeqa/selftest/cases/gcc.py: add libitm to default installed packages.

When running oe-selftest few libitm tests were getting failed due
to below error:
/tmp/runtest.923959/alloc-1.exe: error while loading shared libraries:
libitm.so.1: cannot open shared object file: No such file or directory

Adding libitm to default installed packages to fix this issue.

Signed-off-by: Harish Sadineni <Harish.Sadineni@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>