avoid duplications in the make dependencies

I _suspect_ this caused issues with make failing in dependency tracking
for me, like:

make[2]: Entering directory
'/home/slink/Devel/varnish-git/libvdp-pesi/src'
  CC       vdp_pesi.lo
mv: cannot stat '.deps/vdp_pesi.Tpo': No such file or directory
Makefile:700: recipe for target 'vdp_pesi.lo' failed
make[2]: *** [vdp_pesi.lo] Error 1

src/Makefile.am

@@ -19,7 +19,7 @@ nodist_libvmod_pesi_la_SOURCES = \
 
 dist_man_MANS = vdp_pesi.3
 
-vdp_pesi.lo: $(nodist_libvmod_pesi_la_SOURCES)
+vdp_pesi.lo: vcc_if.h
 
 vcc_if.h vmod_pesi.rst vmod_pesi.man.rst: vcc_if.c