Fix pkg-config file for static linking

[Filip Gospodinov]

Since https://github.com/postgres/postgres/commit/ea53100d5 (or Postgres 
12.0) the shipped pkg-config file is broken for statically linking libpq 
because libpgcommon and libpgport are missing. This patch adds those two 
missing private dependencies.
diff --git a/src/interfaces/libpq/Makefile b/src/interfaces/libpq/Makefile
index 0c4e55b6ad..fe21335d2d 100644
--- a/src/interfaces/libpq/Makefile
+++ b/src/interfaces/libpq/Makefile
@@ -94,7 +94,7 @@ SHLIB_PREREQS = submake-libpgport
 
 SHLIB_EXPORTS = exports.txt
 
-PKG_CONFIG_REQUIRES_PRIVATE = libssl libcrypto
+PKG_CONFIG_REQUIRES_PRIVATE = libpgcommon libpgport libssl libcrypto
 
 all: all-lib