package/gsl: fix powerpc build

Fix the following powerpc build failures on:
 - musl raised because fpu_control.h is not available:

In file included from fp.c:8:
fp-gnuppc.c:21:10: fatal error: fpu_control.h: No such file or directory
   21 | #include <fpu_control.h>
      |          ^~~~~~~~~~~~~~~

 - glibc raised because _FPU_RC_NEAREST is undefined if _SOFT_FLOAT is
   set:

   In file included from fp.c:8:
fp-gnuppc.c: In function 'gsl_ieee_set_mode':
fp-gnuppc.c:53:15: error: '_FPU_RC_NEAREST' undeclared (first use in this function)
   53 |       mode |= _FPU_RC_NEAREST ;
      |               ^~~~~~~~~~~~~~~

These build failures are raised since the addition of the package in
commit 9d9f7feb

Fixes:
 - http://autobuild.buildroot.org/results/d73e116c81bf16d2e55fced215d6bd3b382fef10
 - http://autobuild.buildroot.org/results/48403946bb4cda9013e51db59c1b2ffdcf4e2854



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>