Disable FFT when building in static mode

This is because fftreal's pro file says:
static: error(This library cannot be built for static linkage)
Found due to a CI failure on win32-msvc2010_static_Windows_7.

Change-Id: I2a4630410fa7a1bb4e6ab158a4f7ae76d5cacd8a
Reviewed-by: Holger Ihrig <holger.ihrig@gmail.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

examples/multimedia/spectrum/spectrum.pri

@@ -23,6 +23,8 @@ DEFINES += LOG_ENGINE
 # If this macro is defined, the FFTReal DLL will not be built
 #DEFINES += DISABLE_FFT
 
+static: DEFINES += DISABLE_FFT
+
 # Disables rendering of the waveform
 #DEFINES += DISABLE_WAVEFORM