[Cleanup] Add missing #include. Remove useless ones.

This CL is the result of running include-what-you-use tool on part
of the code base (audio target and dependencies) plus manual fixes.

bug: webrtc:8311
Change-Id: I277d281ce943c3ecc1bd45fd8d83055931743604
Reviewed-on: https://webrtc-review.googlesource.com/c/106280
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25311}

rtc_base/stringutils.h

@@ -20,6 +20,7 @@
 #include <malloc.h>
 #include <wchar.h>
 #include <windows.h>
+
 #define alloca _alloca
 #endif  // WEBRTC_WIN
 
@@ -29,6 +30,7 @@
 #else  // BSD
 #include <alloca.h>
 #endif  // !BSD
+#include <strings.h>
 #endif  // WEBRTC_POSIX
 
 #include <string>