package/rpcbind: fix musl build

package/rpcbind: fix musl build

rpcbind uses the __P() macro defined in sys/cdefs.h but does not
includes it explicitly, relying on the glibc behaviour to include it
from its own headers.

But this macro does nothing but expands as its argument. In the past,
it did provide support for pre-ANSI compilers that did not support
function prototypes. Those are now gone, and glibc provides this macro
for legacy code.

Patch out the use of __P() altogether.

Fixes:
    http://autobuild.buildroot.org/results/346/346c5b562c244ddb039db2f86f71d670a01ed9ce/
    http://autobuild.buildroot.org/results/d77/d77454265c45539178c008c5d92509c441447675/
    and numerous such autobuild failures

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>