Fix CMake warning

1 job for master in 5 minutes and 20 seconds (queued for 23 seconds)
Status Job ID Name Coverage
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failed #17306
build

00:05:20

 
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       inline static int getHorizontalAdvance(const QFontMetrics &metrics, const QChar& c)
^
make[2]: *** [src/CMakeFiles/kdiff3.dir/kdiff3_shell.cpp.o] Error 1
src/CMakeFiles/kdiff3.dir/build.make:86: recipe for target 'src/CMakeFiles/kdiff3.dir/kdiff3_shell.cpp.o' failed
CMakeFiles/Makefile2:291: recipe for target 'src/CMakeFiles/kdiff3.dir/all' failed
make[1]: *** [src/CMakeFiles/kdiff3.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2
ERROR: Job failed: exit code 1