Building against PG 12 is broken

[Algunenano]

Example from debbie (​https://debbie.postgis.net/job/PostGIS_Regress/11208/console):

gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -O2 -fPIC -I../liblwgeom -std=gnu99 -g -O2 -fno-math-errno -fno-signed-zeros -I../libpgcommon  -I/var/lib/jenkins/workspace/geos/rel-3.7w64/include   -I/usr/include/libxml2 -I/usr/include -DHAVE_SFCGAL    -fPIC -I/usr/include -DHAVE_SFCGAL -I. -I./ -I/var/lib/jenkins/workspace/pg/rel/pg12w64/include/postgresql/server -I/var/lib/jenkins/workspace/pg/rel/pg12w64/include/postgresql/internal  -D_GNU_SOURCE   -c -o gserialized_estimate.o gserialized_estimate.c
gserialized_estimate.c:81:10: fatal error: utils/tqual.h: No such file or directory
 #include "utils/tqual.h"
          ^~~~~~~~~~~~~~~
compilation terminated.
make[1]: *** [<builtin>: gserialized_estimate.o] Error 1

Changed in the following PG commit:

commit c91560defc57f89f7e88632ea14ae77b5cec78ee
Author: Andres Freund <andres@anarazel.de>
Date:   Mon Jan 21 17:03:15 2019 -0800

    Move remaining code from tqual.[ch] to heapam.h / heapam_visibility.c.
    
    Given these routines are heap specific, and that there will be more
    generic visibility support in via table AM, it makes sense to move the
    prototypes to heapam.h (routines like HeapTupleSatisfiesVacuum will
    not be exposed in a generic fashion, because they are too storage
    specific).
    
    Similarly, the code in tqual.c is specific to heap, so moving it into
    access/heap/ makes sense.
    
    Author: Andres Freund
    Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Should be backported as far as we want to support PG12 (2.4?)

[Algunenano]

I've removed that header inclusion in r17214. If that makes the bots happy I'll backport it to 2.5 (2.4 doesn't include it).

[algunenano]

In 17215:

PG12 compat: Remove unnecessary header include (tqual.h)

Closes #4307