Dynamic modules: added dlsym() to configure test.

It appears that AddressSanitizer in GCC 5 intercepts dlopen() and dlclose()
but not dlsym().

auto/unix

diff --git a/auto/unix b/auto/unix
index 6ca63a6..e07df08 100755
--- a/auto/unix
+++ b/auto/unix
@@ -260,7 +260,7 @@
 ngx_feature_incs="#include <dlfcn.h>"
 ngx_feature_path=
 ngx_feature_libs=
-ngx_feature_test="dlopen(NULL, 0)"
+ngx_feature_test="dlopen(NULL, 0); dlsym(NULL, NULL)"
 . auto/feature