goldfish-opengl: add cstring in various places

Helps with the g++ build.  Also, remove $(LINUX_GUEST_BUILD)
guard since it's not useful.

BUG=202552093
TEST=compile

Change-Id: I40680660a3f2b4da97be61c8b827cd20b087ea95
diff --git a/system/OpenglSystemCommon/HostConnection.h b/system/OpenglSystemCommon/HostConnection.h
index 189b8bf..a6810a5 100644
--- a/system/OpenglSystemCommon/HostConnection.h
+++ b/system/OpenglSystemCommon/HostConnection.h
@@ -35,7 +35,7 @@
 #endif
 
 #include <memory>
-#include <string>
+#include <cstring>
 
 class GLEncoder;
 struct gl_client_context_t;