selftests/bpf: Fix liburandom_read.so linker error

When building with O=, the following linker error occurs:

    clang: error: no such file or directory: 'liburandom_read.so'

Fix by adding $(OUTPUT) to the linker search path.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-5-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -189,7 +189,7 @@ $(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
 $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
 	$(call msg,BINARY,,$@)
 	$(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
-		     liburandom_read.so $(filter-out -static,$(LDLIBS))	     \
+		     -lurandom_read $(filter-out -static,$(LDLIBS)) -L$(OUTPUT)  \
 		     -fuse-ld=$(LLD) -Wl,-znoseparate-code -Wl,--build-id=sha1 \
 		     -Wl,-rpath=. -o $@