Merge pull request #2015 from yshui/musl_libc_src_core_sys_posix_dlfcn

 Add musl libc definitions to:  src_core_sys_posix_dlfcn
merged-on-behalf-of: Sebastian Wilzbach <sebi.wilzbach@gmail.com>

Author: dlang-bot

src/core/sys/posix/dlfcn.d

@@ -291,3 +291,18 @@ else version( CRuntime_Bionic )
         void*        dli_saddr;
     }
 }
+else version( CRuntime_Musl )
+{
+    enum {
+        RTLD_LAZY     = 1,
+        RTLD_NOW      = 2,
+        RTLD_NOLOAD   = 4,
+        RTLD_NODELETE = 4096,
+        RTLD_GLOBAL   = 256,
+        RTLD_LOCAL    = 0,
+    }
+    int          dlclose(void*);
+    const(char)* dlerror();
+    void*        dlopen(in char*, int);
+    void*        dlsym(void*, in char*);
+}