base: Avoid an unused expression error in GEM5_DEPRECATED_MACRO.

If the lambda in that macro is not actually called, clang complains
about an unused expression which breaks the build.

Change-Id: Id8708944cbac4c937e833583da0c2c18422230c0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45719
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/compiler.hh

@@ -132,7 +132,7 @@
 // The definition must be an c++ expression and not a statement because of how
 // the original macro is wrapped.
 #  define GEM5_DEPRECATED_MACRO(name, definition, message) \
-     ([](){GEM5_DEPRECATED(message) int name{}; return name;}, (definition))
+     ([](){GEM5_DEPRECATED(message) int name{}; return name;}(), (definition))
 // This version is for macros which are statement-like, which frequently use
 // "do {} while (0)" to make their syntax look more like normal c++ statements.
 #  define GEM5_DEPRECATED_MACRO_STMT(name, definition, message) \