Make pw_build_host_tools a build arg in host build

This defines pw_build_host_tools as a GN build arg, allowing it to be
toggled without changing the host target.

Change-Id: I9b053b26e4deffc04885015c6981496bb857ee33
diff --git a/targets/host/host_common.gni b/targets/host/host_common.gni
index b94023b..543268b 100644
--- a/targets/host/host_common.gni
+++ b/targets/host/host_common.gni
@@ -17,4 +17,6 @@
 # Configure backend for pw_dumb_io facade.
 dir_pw_dumb_io_backend = "$dir_pw_dumb_io_stdio"
 
-pw_build_host_tools = true
+declare_args() {
+  pw_build_host_tools = true
+}