qt5/qt5base: Fix build on MIPS without DSP support

The qt_memfill32_asm_mips_dsp function is only declared if
QT_COMPILER_SUPPORTS_MIPS_DSP is defined, so we can't reference it
unless the same macro is defined.

Backporting an upstream patch to fix this issue.

This is the error message:

qt-everywhere-enterprise-src-5.4.0-alpha/qtbase/src/gui/painting/qdrawhelper.cpp:
In function 'void qInitDrawhelperAsm()':
qt-everywhere-enterprise-src-5.4.0-alpha/qtbase/src/gui/painting/qdrawhelper.cpp:6843:
error: 'qt_memfill32_asm_mips_dsp' was not declared in this scope

Upstream commit:

  https://qt.gitorious.org/qt/qtbase/commit/2ca323ccd4f25f409eff8fc0c9804099b78c2bde

Fixes:

  http://autobuild.buildroot.net/results/254/254cf62bef8bc5d5ffe345d817d16aa9983baa15/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>