bazel windows: unbreak MallocExtension visibility

BUILD.bazel

diff --git a/BUILD.bazel b/BUILD.bazel
index 84fe9bf..c84fbe5 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -164,6 +164,7 @@
         "src/gperftools/nallocx.h",
         "src/gperftools/tcmalloc.h",
     ],
+    defines = ["PERFTOOLS_DLL_DECL="],
     copts = CXXFLAGS,
     includes = [
         "generic-config",

generic-config/config.h

diff --git a/generic-config/config.h b/generic-config/config.h
index a6f65f4..73d9fd2 100644
--- a/generic-config/config.h
+++ b/generic-config/config.h
@@ -263,7 +263,9 @@
 #endif
 
 /* dllexport or attribute visibility */
+#ifndef PERFTOOLS_DLL_DECL
 #define PERFTOOLS_DLL_DECL /**/
+#endif
 
 /* if libgcc stacktrace method should be default */
 /* #undef PREFER_LIBGCC_UNWINDER */