add src/firehose/firehose_internal.h so distcheck rule will work on Linux

Author: dgrove-oss

src/Makefile.am

@@ -40,6 +40,7 @@ libdispatch_la_SOURCES=		\
 	source_internal.h		\
 	trace.h					\
 	voucher_internal.h		\
+	firehose/firehose_internal.h \
 	shims/atomic.h			\
 	shims/atomic_sfb.h		\
 	shims/getprogname.h		\