Install JIT related headers.

There's no reason not to install these, and jit.h can be useful for
users of e.g. planner hooks.

Author: Donald Dong
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/296D405F-7F95-49F1-B565-389D6AA78505@csumb.edu
Backpatch: 11-, where JIT compilation was introduced

src/include/Makefile

@@ -18,7 +18,7 @@ all: pg_config.h pg_config_ext.h pg_config_os.h
 
 # Subdirectories containing installable headers
 SUBDIRS = access bootstrap catalog commands common datatype \
-	executor fe_utils foreign \
+	executor fe_utils foreign jit \
 	lib libpq mb nodes optimizer parser partitioning postmaster \
 	regex replication rewrite \
 	statistics storage tcop snowball snowball/libstemmer tsearch \