meson: Fix xkbcommon-x11.pc Requires versioning

meson: Fix xkbcommon-x11.pc Requires versioning

Old meson expects an array with one dependency per element. Providing a
string containing multiple deps results in only the first dep getting
its whitespace properly applied. As a result, the output was:

    Requires.private: xcb >= 1.10 xcb-xkb>=1.10

And downstream projects failed to find a package named 'xcb-xkb>=1.10'.

Specifying an array of versioned deps results in correct output:

    Requires.private: xcb >= 1.10, xcb-xkb >= 1.10

Fixes #64.

Signed-off-by: Jan Alexander Steffens (heftig) <jan.steffens@gmail.com>