Skip utmp_test.cpp for musl

Musl #defines utmp to utmpx, which causes the tests in utmp_test.cpp
to collide with the ones in utmpx_test.cpp.  Skip the utmp_test.cpp
when building for musl.

Test: m USE_HOST_MUSL=true
Change-Id: Ie05ddf6e1fd8c3bcc687d2537c19b6ab70ab8d67
diff --git a/tests/Android.bp b/tests/Android.bp
index 1949079..da1df12 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -541,6 +541,10 @@
 
                 // unsupported relocation type 37
                 "ifunc_test.cpp",
+
+                // musl #defines utmp to utmpx, causing a collision with
+                // utmpx_test.cpp
+                "utmp_test.cpp",
             ],
         },
     },