Travis O3 run failing on gserialized_gist_2d.c

[robe]

Since commit r16626

Travis O3 run has been failing with:

​https://travis-ci.org/postgis/postgis/jobs/398832331

gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -fPIC -pie -fno-omit-frame-pointer -fPIC -I../liblwgeom -g -O3 -mtune=generic -fno-omit-frame-pointer -Werror -Wall -Wextra -Wno-unused-parameter -I../libpgcommon  -I/usr/include    -I/usr/include/libxml2 -I/usr/include -DHAVE_SFCGAL    -fPIC -I/usr/include -DHAVE_SFCGAL -I. -I./ -I/usr/include/postgresql/9.6/server -I/usr/include/postgresql/internal -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2  -I/usr/include/mit-krb5  -c -o gserialized_gist_2d.o gserialized_gist_2d.c
gserialized_gist_2d.c:164:13: error: no previous prototype for ‘box2df_set_empty’ [-Werror=missing-prototypes]
 inline void box2df_set_empty(BOX2DF *a)
             ^
gserialized_gist_2d.c:170:13: error: no previous prototype for ‘box2df_set_finite’ [-Werror=missing-prototypes]
 inline void box2df_set_finite(BOX2DF *a)
             ^
gserialized_gist_2d.c:332:13: error: no previous prototype for ‘box2df_validate’ [-Werror=missing-prototypes]
 inline void box2df_validate(BOX2DF *b)
             ^
gserialized_gist_2d.c:355:6: error: no previous prototype for ‘box2df_overlaps’ [-Werror=missing-prototypes]
 bool box2df_overlaps(const BOX2DF *a, const BOX2DF *b)
      ^
gserialized_gist_2d.c:400:6: error: no previous prototype for ‘box2df_equals’ [-Werror=missing-prototypes]
 bool box2df_equals(const BOX2DF *a, const BOX2DF *b)
      ^
gserialized_gist_2d.c:416:6: error: no previous prototype for ‘box2df_overleft’ [-Werror=missing-prototypes]
 bool box2df_overleft(const BOX2DF *a, const BOX2DF *b)

This is a direct result of having taken out the statics, so guess we need a header definition for each now.

[robe]

In 16629:

Get rid of the ifdefs around the functions, these have always existed but were static before.
Closes #4114