config/ppc: fix build with GCC >= 10

config/ppc: fix build with GCC >= 10

Like for python, multiline statements in meson must either use a
backslash character (explicit continuation) or be enclosed in ()
(implicit continuation).

python PEP8 recommends the latter [1], and it looks like meson had
an issue with backslash before 0.50 [2].

1: https://www.python.org/dev/peps/pep-0008/#multiline-if-statements
2: https://github.com/mesonbuild/meson/commit/90c9b868b20b

Fixes: 394407f50c90 ("config/ppc: ignore GCC 11 psabi warnings")

Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>