build: Use Link Time Optimization for Qt code on Linux

See: https://www.qt.io/blog/2019/01/02/qt-applications-lto

Author: hebasto

depends/packages/qt.mk

@@ -152,6 +152,9 @@ $(package)_config_opts_linux += -fontconfig
 $(package)_config_opts_linux += -no-opengl
 $(package)_config_opts_linux += -no-feature-vulkan
 $(package)_config_opts_linux += -dbus-runtime
+ifneq ($(LTO),)
+$(package)_config_opts_linux += -ltcg
+endif
 $(package)_config_opts_arm_linux += -platform linux-g++ -xplatform bitcoin-linux-g++
 $(package)_config_opts_i686_linux  = -xplatform linux-g++-32
 ifneq (,$(findstring -stdlib=libc++,$($(1)_cxx)))