x11-wm/awesome: Fix build with GCC-based architectures

This port requires C11-aware compiler.
Add USES=compiler:c11 to fix build on GCC architectures.

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PR: 232855 [1]
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl> [1]
Approved by: garga (maintainer, IRC)
Sponsored by: IntegriCloud [1]