multiprocess build fix: ipc/capnp/init.capnp.h: No such file or directory

Error was reported by SatoriHoshiAiko in
bitcoin#25207 and happens unpredictably
because make doesn't always build dependencies in the same order.

The source file src/ipc/capnp/protocol.cpp includes some generated headers so
needs to have an explicit dependency specified in the makefile so the headers
will be generated before the file is compiled. bitcoin#19160 added the explicit
dependency, but it was incorrect because it referred to an old file path from
before the source file was renamed (ipc.cpp -> protocol.cpp)

Author: ryanofsky

src/Makefile.am

@@ -330,7 +330,7 @@ obj/build.h: FORCE
 	  "$(abs_top_srcdir)"
 libbitcoin_util_a-clientversion.$(OBJEXT): obj/build.h
 
-ipc/capnp/libbitcoin_ipc_a-ipc.$(OBJEXT): $(libbitcoin_ipc_mpgen_input:=.h)
+ipc/capnp/libbitcoin_ipc_a-protocol.$(OBJEXT): $(libbitcoin_ipc_mpgen_input:=.h)
 
 # server: shared between bitcoind and bitcoin-qt
 # Contains code accessing mempool and chain state that is meant to be separated