[build] Fix building on older versions of gcc

Versions of gcc prior to 9.1 do not support the single-argument form
of static_assert().  Fix by unconditionally defining a compatibility
macro for the single file that uses this.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/linux/linux_api.c

@@ -40,6 +40,9 @@
 #include <slirp/libslirp.h>
 #endif
 
+#undef static_assert
+#define static_assert(x) _Static_assert(x, #x)
+
 /** @file
  *
  * Linux host API