[libc][obvious] fix strdup being listed twice

strdup was being included even if malloc wasn't and that was causing
a build failure.

Differential Revision: https://reviews.llvm.org/D112641

Author: michaelrj-google

libc/config/linux/x86_64/entrypoints.txt

@@ -36,7 +36,6 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
-    libc.src.string.strdup
     libc.src.string.strlen
     libc.src.string.strncat
     libc.src.string.strncmp