pgsql: Add missed src/include/foreign subdirectory to the set installed

Log Message:
-----------
Add missed src/include/foreign subdirectory to the set installed into
INSTALLDIR/include/server/. Itagaki Takahiro

Tags:
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REL8_4_STABLE

Modified Files:
--------------
pgsql/src/include:
Makefile (r1.26 -> r1.26.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/Makefile?r1=1.26&r2=1.26.2.1)