Fix -Wreturn-type warning for jpeg version check

Summary: See also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87515

Test Plan:
Build gwenview with '-Werror=return-type' in CFLAGS/CXXFLAGS.
Without this patch cmake call should fail if clean build directory is used.
With this patch applied build should succeed.

Reviewers: ngraham, aacid

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D19993