bug#43135: 28.0.50; image.c compiler warning

[David Fussner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

For the last week or so I'm seeing this warning when compiling master,
always using the simple command 'make' in the git repo:

  CC       image.o
image.c: In function 'svg_load_image':
image.c:9892:3: warning: implicit declaration of function 'free'
[-Wimplicit-function-declaration]
   free (wrapped_contents);
   ^
image.c:9892:3: warning: incompatible implicit declaration of built-in
function 'free'
image.c:9892:3: note: include '<stdlib.h>' or provide a declaration of
'free'

This is a 32-bit build, and Slackware 14.2's gcc is 5.5.0. The
executable runs just fine.

The warning appears after commit
8f42b94fe43911c6b0c7e519ba439d61459dc744 by Alan Third:
Set basic SVG attributes (bug#40845)

Including <stdlib.h> at the top of image.c does silence the warning.

If my toolchain is outdated please forgive and ignore the noise.

Thanks,

David.

In GNU Emacs 28.0.50 (build 36, i686-pc-linux-gnu, GTK+ Version
3.18.9, cairo version 1.14.6)
 of 2020-08-31 built on newfont
Repository revision: 58d962379546762c7eead8fc69c4c136bed8256b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description: Slackware 14.2

Configured using:
 'configure
 PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 XDBE XIM MODULES THREADS PDUMPER LCMS2

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.ISO8859-1
  locale-coding-system: iso-latin-1-unix