nmap: remove redundant --static flag to pkg-config

The pkg-config tool is automatically passing --static when BR2_STATIC_LIBS
is set (see package/pkgconf/pkgconf.mk), so no need to pass it explicitly
from package .mk files.

Additionally, replace a missed occurrence of $(shell) with backticks.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>