lib/math: add missing MODULE_DESCRIPTION() macros

lib/math: add missing MODULE_DESCRIPTION() macros

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/math/prime_numbers.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/math/rational-test.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240531-md-lib-math-v1-1-11a3bec51ebb@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>