scons: Use the 'png' tag for the PNG library.

Use that instead of checking HAVE_PNG manually.

Change-Id: I70876982c2506bee1173341fa100f5fae2ae4302
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58709
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/SConscript

@@ -43,8 +43,7 @@ Executable('cprintftime', 'cprintftime.cc', 'cprintf.cc')
 Source('debug.cc', add_tags=['gem5 trace', 'gem5 events'])
 GTest('debug.test', 'debug.test.cc', 'debug.cc')
 Source('fenv.cc', tags='fenv')
-if env['CONF']['HAVE_PNG']:
-    SourceLib('png')
+SourceLib('png', tags='png')
 Source('pngwriter.cc', tags='png')
 Source('fiber.cc')
 GTest('fiber.test', 'fiber.test.cc', 'fiber.cc')