libfreeimage: fix build issue with gcc 5.x

Fixes:
http://autobuild.buildroot.net/results/fe9/fe9c5ee9cdc52c48df5efe7560ccd5a0d3cde154/
http://autobuild.buildroot.net/results/358/3580d520da6f8d8f4ffeffcd2b969ebb0851ad37/
http://autobuild.buildroot.net/results/0cb/0cb2f0c69301c7a85a354c435da245cd904533fb/
http://autobuild.buildroot.net/results/52c/52c2967015aa79843db05e901a1563447a4029b2/

gcc 5.x gets confused by the string literals used in the inline assembly in
the files from the embedded libwebp library.  This issue is already fixed in
upstream libwebp, but that fix is not directly applicable to the tweaked version
embedded in libfreeimage.

I haven't been able to find a public VCS for libfreeimage, but the issue has
been discussed on the freeimage list:

http://sourceforge.net/p/freeimage/discussion/36110/thread/605ef8e4/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>