Fix undefined references seen with mingw-w64

	* config/abi/pre/gnu.ver: Add missing exports for mingw.

From-SVN: r263977

libstdc++-v3/ChangeLog

 2018-08-30  Jonathan Wakely  <jwakely@redhat.com>
 
+	* config/abi/pre/gnu.ver: Add missing exports for mingw.
+
 	* include/ext/pointer.h (_Pointer_adapter): Define operators for
 	pointer arithmetic using long long offsets.
 	* testsuite/ext/ext_pointer/1.cc: Test pointer arithmetic using

libstdc++-v3/config/abi/pre/gnu.ver

@@ -2049,6 +2049,11 @@ GLIBCXX_3.4.26 {
     # std::__throw_ios_failure(const char*, int);
     _ZSt19__throw_ios_failurePKci;
 
+    # std::wistream::ignore(long long, unsigned short)
+    _ZNSt13basic_istreamIwSt11char_traitsIwEE6ignoreExt;
+    # std::basic_filebuf::open(const wchar_t*, openmode)
+    _ZNSt13basic_filebufI[cw]St11char_traitsI[cw]EE4openEPKwSt13_Ios_Openmode;
+
 } GLIBCXX_3.4.25;
 
 # Symbols in the support library (libsupc++) have their own tag.