liquid-dsp: wrapping isnan in T_ABS to help compilation w/ certain gcc

liquid-dsp: wrapping isnan in T_ABS to help compilation w/ certain gcc

Backporting an upstreamed patch in order to fix a build failure like
this one:

In file included from src/math/src/polyc.c:43:0:
src/math/src/poly.findroots.c: In function
'polyc_findroots_bairstow_recursion':
src/math/src/poly.findroots.c:305:9: error: non-floating-point argument
in call to function '__builtin_isnan'
         if (isnan(du) || isnan(dv)) {

Upstream commit URL:

  https://github.com/jgaeddert/liquid-dsp/commit/3055eb3da9d0a202c1a975f7db0c8370a09a30bc

Fixes:

  http://autobuild.buildroot.net/results/01d/01d7d4f34b256bcdf30b16180c015f146bd50e63/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[Thomas: tweak patch format.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>