Add missing "-I." flag when building pg_bsd_indent.

This is evidently not required by most compilers, but buildfarm
member fairywren is unhappy without it.  It looks like the meson
infrastructure has this right already.

Author: tglsfdc

src/tools/pg_bsd_indent/Makefile

@@ -13,6 +13,8 @@ subdir = src/tools/pg_bsd_indent
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
+override CPPFLAGS := -I$(srcdir) $(CPPFLAGS)
+
 OBJS = \
 	$(WIN32RES) \
 	args.o \