pdfium: disable custom allocator

That causes problems on aarch64 (see
<https://lists.freedesktop.org/archives/libreoffice/2018-February/079384.html>)
and provides little benefit to our use-case.

Change-Id: I133d41476ab84cec67abc5b36a1e16c91bc6ac15
Reviewed-on: https://gerrit.libreoffice.org/49684Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

external/pdfium/Library_pdfium.mk

@@ -24,6 +24,7 @@ $(eval $(call gb_Library_add_defs,pdfium,\
     -DUSE_SYSTEM_LCMS2 \
     -DUSE_SYSTEM_LIBJPEG \
     -DUSE_SYSTEM_ZLIB \
+    -DMEMORY_TOOL_REPLACES_ALLOCATOR \
     -DUNICODE \
 ))