eal/windows: fix MinGW build

eal/windows: fix MinGW build

the strncasecmp macro defined in rte_os_shim.h is already
defined in MinGW-w64, as a result the compiler prints out
the warning below on function redefinition whenever compiling
a file including the header in debug mode.

lib/eal/windows/include/rte_os_shim.h:21:
warning: "strncasecmp" redefined

Fixed by defining the macro only to the clang compiler.

Fixes: 45d62067c237 ("eal: make OS shims internal")

Signed-off-by: Tal Shnaiderman <talshn@nvidia.com>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>