Tests fail to build with gcc-14 (-Werror=implicit-function-declaration)

[orlitzky]

With gcc-14, implicit function definitions have become an error by default. There are a few of these in the test suite:

/home/mjo/src/sleef.git/src/libm-tester/tester2simdsp.c: In function 'main':
/home/mjo/src/sleef.git/src/libm-tester/tester2simdsp.c:895:54: error: implicit declaration of function 'isinff' [-Wimplicit-function-declaration]
  895 |           (d >=  sqrt(FLT_MAX) && !(u0 <= 1.0001 || (isinff(t) && t > 0))) ||

/home/mjo/src/sleef.git/src/libm-tester/tester2ld.c: In function 'countULP':
/home/mjo/src/sleef.git/src/libm-tester/tester2ld.c:56:7: error: implicit declaration of function 'isnanl' [-Wimplicit-function-declaration]
   56 |   if (isnanl(c2) && isnanl(d)) return 0;

/home/mjo/src/sleef.git/src/libm-tester/tester2sp.c: In function 'main':
/home/mjo/src/sleef.git/src/libm-tester/tester2sp.c:657:54: error: implicit declaration of function 'isinff' [-Wimplicit-function-declaration]
  657 |           (d >=  sqrt(FLT_MAX) && !(u0 <= 1.0001 || (isinff(t) && t > 0))) ||

I copy/pasted some things to get it working, but usually the issue comes down to a missing #include.