Fix Ardour VAMP plugins (FFT)

Fix Ardour VAMP plugins (FFT)

Bug was introduced in 8ed33f1bc714c1 symbol visibility setting
in CFLAGS, CXXFLAGS was overridden. This resulted in publicly exposed
and bound kiss_fft symbols in libqm-dsp.

At runtime those symbols were resolved using previously bound
symbols in libcodec  (see below) that uses a mismatching implementation
(ardour/qm-dsp uses -Dkiss_fft_scalar=double)

#0  0x00007fffea793d40 in kiss_fftr () at /usr/lib/x86_64-linux-gnu/libcodec2.so.0.8.1
#1  0x00007fffcf4516ab in FFTReal::D::forward(double const*, double*, double*) (this=0x5555571d73a0, ri=0x55555a734810, ro=0x55555a7262b0, io=0x55555a728590)
    at ../libs/qm-dsp/dsp/transforms/FFT.cpp:121
#2  0x00007fffcf4510fd in FFTReal::forward(double const*, double*, double*) (this=0x555559868190, ri=0x55555a734810, ro=0x55555a7262b0, io=0x55555a728590)
    at ../libs/qm-dsp/dsp/transforms/FFT.cpp:186