opal.pc.in: fix typo; use the write AC_SUBST'ed variable

As reported by @marksantcroos, this substitution in opal.pc was
incorrect -- it left @{libdir} in the string (vs. ${libdir}).  The fix
is simple: use the proper substitution variable in opal.pc (it was
never updated to reflect the new/correct name that was created just
for the pkg-config files).

Fixes open-mpi/ompi#1343.

opal/tools/wrappers/opal.pc.in

@@ -14,7 +14,7 @@ libdir=@libdir@
 # (they're pulled in via libopen-pal.so's implicit dependencies), so
 # list them in Libs.private.
 #
-Libs: -L${libdir} @OPAL_WRAPPER_EXTRA_LDFLAGS@ -lopen-pal
+Libs: -L${libdir} @OPAL_PKG_CONFIG_LDFLAGS@ -lopen-pal
 Libs.private: @OPAL_WRAPPER_EXTRA_LIBS@
 #
 # It is safe to hard-wire the -I before the EXTRA_INCLUDES because we