libtirpc: fix musl build

Don't redefine existing typedef (u_char, u_long, ...) on musl

Fixes:
 - http://autobuild.buildroot.net/results/dbc07e383605a84eb19a2fd1899668612212518a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>