Fixed compilation if GCC LTO enabled.

[fedan]

Hello!
I tested it on dev-qt / qtwebkit-5.7.1 in Calculate Linux x86_64 (Gentoo) and gcc-5.4.0.
Patched 3 branches: 5.7, 5.8, 5.9.
My config for LTO:

CC="/usr/bin/gcc"
CXX="/usr/bin/g++"
AR="/usr/bin/gcc-ar"
NM="/usr/bin/gcc-nm"
RANLIB="/usr/bin/gcc-ranlib"
LD="/usr/bin/ld.gold"
CFLAGS="-O3 -march=native -fomit-frame-pointer -pipe -flto=4 -fuse-linker-plugin -fuse-ld=gold"
CXXFLAGS="${CFLAGS}"
LDFLAGS="${CFLAGS} -Wl,--sort-common,--hash-style=gnu,--as-needed"

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