Mark declarations of externally instantiated functions as inline so GCC doesn't complain.

llvm-svn: 255599
diff --git a/libcxx/include/valarray b/libcxx/include/valarray
index ecfd75b..bde644e 100644
--- a/libcxx/include/valarray
+++ b/libcxx/include/valarray
@@ -803,7 +803,7 @@
     _LIBCPP_INLINE_VISIBILITY
     valarray() : __begin_(0), __end_(0) {}
     _LIBCPP_INLINE_VISIBILITY
-    explicit valarray(size_t __n);
+    inline explicit valarray(size_t __n);
     _LIBCPP_INLINE_VISIBILITY
     valarray(const value_type& __x, size_t __n);
     valarray(const value_type* __p, size_t __n);
@@ -819,7 +819,7 @@
     valarray(const gslice_array<value_type>& __ga);
     valarray(const mask_array<value_type>& __ma);
     valarray(const indirect_array<value_type>& __ia);
-    _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY
     ~valarray();
 
     // assignment: