spell out dependencies on vmod_re2.h -> vcc_if.h

Fixes parallel build failures when compilation of vmod_re2.c,
set.c or re2.c is attempted before vmodtool is finished

src/Makefile.am

@@ -25,6 +25,8 @@ MAINTAINERCLEANFILES = $(dist_man_MANS)
 
 libvmod_re2_la_LIBADD = @RE2_LIBS@
 
+vmod_re2.c set.c re2.c: vcc_if.h
+
 vmod_re2.lo: $(nodist_libvmod_re2_la_SOURCES)
 
 vcc_if.h vmod_re2.rst vmod_re2.man.rst: vcc_if.c