headerscheck: Document that --with-llvm is required

We already documented that other --with-* options are required for a
successful run.  It turns out --with-llvm is also required.

Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/1127775.1754417387%40sss.pgh.pa.us

src/tools/pginclude/README

@@ -55,8 +55,12 @@ and are skipped by the headerscheck script.
 
 The easy way to run the script is to say "make -s headerscheck" in
 the top-level build directory after completing a build.  You should
-have included "--with-perl --with-python" in your configure options,
-else you're likely to get errors about related headers not being found.
+have included at least
+
+    --with-llvm --with-perl --with-python
+
+in your configure options, else you're likely to get errors about
+related headers not being found.
 
 A limitation of the current script is that it doesn't know exactly which
 headers are for frontend or backend; when in doubt it uses postgres.h as
@@ -78,8 +82,12 @@ and are skipped by the script in the --cplusplus mode.
 
 The easy way to run the script is to say "make -s cpluspluscheck" in
 the top-level build directory after completing a build.  You should
-have included "--with-perl --with-python" in your configure options,
-else you're likely to get errors about related headers not being found.
+have included at least
+
+    --with-llvm --with-perl --with-python
+
+in your configure options, else you're likely to get errors about
+related headers not being found.
 
 If you are using a non-g++-compatible C++ compiler, you may need to
 override the script's CXXFLAGS setting by setting a suitable environment