build: fix libm detection in meson

build: fix libm detection in meson

Using version 0.47.1, meson is unable to find the math library in Travis
for the 32bits job.
Quite surprisingly, this problem is not seen with the 64bits jobs.

Switching to 0.48.0, the problem disappears.

But we should pass 'm' to find_library instead of 'libm' anyway.

Fixes: 98edcbb5ab2f ("eal/windows: introduce Windows support")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>