remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/LogInit.cxx

@@ -46,7 +46,7 @@
 #define LOG_DATE_BUF_SIZE 16
 #define LOG_DATE_LEN (LOG_DATE_BUF_SIZE - 1)
 
-gcc_unused
+[[maybe_unused]]
 static constexpr Domain log_domain("log");
 
 #ifndef ANDROID