Win32: fixed link flags with MSVC, broken by bfe536716dbf.

Notably, "-debug" was omitted, resulting in an executable without debug
symbols.

auto/cc/msvc

diff --git a/auto/cc/msvc b/auto/cc/msvc
index 6cb8b3d..393ba32 100644
--- a/auto/cc/msvc
+++ b/auto/cc/msvc
@@ -106,7 +106,7 @@
 
 # precompiled headers
 CORE_DEPS="$CORE_DEPS $NGX_OBJS/ngx_config.pch"
-CORE_LINK="$NGX_OBJS/ngx_pch.obj"
+CORE_LINK="$CORE_LINK $NGX_OBJS/ngx_pch.obj"
 NGX_PCH="$NGX_OBJS/ngx_config.pch"
 NGX_BUILD_PCH="-Ycngx_config.h -Fp$NGX_OBJS/ngx_config.pch"
 NGX_USE_PCH="-Yungx_config.h -Fp$NGX_OBJS/ngx_config.pch"