Merge bitcoin/bitcoin#25424: build, qt: Fix QMAKE_CXXFLAGS expression for mingw32 host

71d33bf build, qt: Fix `QMAKE_CXXFLAGS` expression for `mingw32` host (Hennadii Stepanov)

Pull request description:

  A "copy-paste typo" was introduced in bitcoin/bitcoin#21593.

  I'm sorry about that.

ACKs for top commit:
  laanwj:
    Code review ACK 71d33bf

Tree-SHA512: 9433d9e51d7772d4ac1579aab75d184bb486e575ec216933b1a5171b0b68718943c96bd3669fe239b1d1ed17ec82bc5d14e777d4ff8b6fae370352864da88534

Author: laanwj

depends/packages/qt.mk

@@ -171,7 +171,7 @@ $(package)_config_opts_mingw32 += -no-freetype
 $(package)_config_opts_mingw32 += -xplatform win32-g++
 $(package)_config_opts_mingw32 += "QMAKE_CFLAGS = '$($(package)_cflags) $($(package)_cppflags)'"
 $(package)_config_opts_mingw32 += "QMAKE_CXX = '$($(package)_cxx)'"
-$(package)_config_opts_mingw32 += "QMAKE_CXXFLAGS = '$($(package)_cflags) $($(package)_cppflags)'"
+$(package)_config_opts_mingw32 += "QMAKE_CXXFLAGS = '$($(package)_cxxflags) $($(package)_cppflags)'"
 $(package)_config_opts_mingw32 += "QMAKE_LFLAGS = '$($(package)_ldflags)'"
 $(package)_config_opts_mingw32 += -device-option CROSS_COMPILE="$(host)-"
 $(package)_config_opts_mingw32 += -pch