madplay: fixes build when toolchain has locale support

Otherwise build fails in intl/relocatable.c with:
./relocatable.c: In function 'libintl_relocate':
./relocatable.c:402:40: error: 'INSTALLPREFIX' undeclared (first use in this function)
./relocatable.c:402:40: note: each undeclared identifier is reported only once for each function it appears in

[Peter: drop redundant 'else' part]
Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>