src/Makefile.am, bug #327, "fix install path for include files"

install in $includedir/ngspice
which is by default $prefix/include/ngspice

Thanks to Maciej Suminski who reported the issue in
  #327 Header files are installed  in /usr/share/ngspice/include/ngspice
  https://sourceforge.net/p/ngspice/bugs/327/

src/include/ngspice/Makefile.am

@@ -1,11 +1,9 @@
 ## Process this file with automake to produce Makefile.in
 
-includedir = $(pkgdatadir)/include/ngspice
-
-nodist_include_HEADERS = \
+nodist_pkginclude_HEADERS = \
 	config.h
 
-include_HEADERS = \
+pkginclude_HEADERS = \
 	tclspice.h	\
 	acdefs.h	\
 	bdrydefs.h	\