refactor: move run_command from util to common

Quoting ryanofsky: "util can be the library for things included in the kernel
which the kernel can depend on, and common can be the library for other code
that needs to be shared internally, but should not be part of the kernel or
shared externally."

src/external_signer.cpp

@@ -3,9 +3,9 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <chainparams.h>
+#include <common/run_command.h>
 #include <core_io.h>
 #include <psbt.h>
-#include <util/run_command.h>
 #include <util/strencodings.h>
 #include <external_signer.h>