package/brltty: fix build with expat

package/brltty: fix build with expat

tbl2hex is a host command line that is built with:

$(CC_FOR_BUILD) $(LDFLAGS_FOR_BUILD) -o $@ $(TBL2HEX_OBJECTS) $(EXPAT_LIBS_FOR_BUILD) $(LDLIBS_FOR_BUILD)

It needs cldr.o which can depends on expat and is built through:

%.$B: $(SRC_DIR)/%.c
       $(CC_FOR_BUILD) -DFOR_BUILD $(CFLAGS_FOR_BUILD) -o $@ -c $<

When cross-compiling, build fails because expat is not found on host:

gcc -DFOR_BUILD -I. -I. -I./../Programs -I../Programs -I../Headers -I./.. -I.. -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=2 -D_BSD_SOURCE -D_XOPEN_SOURCE=500 -D_XOPEN_SOURCE_EXTENDED -D_GNU_SOURCE  -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=2 -D_BSD_SOURCE -D_XOPEN_SOURCE=500 -D_XOPEN_SOURCE_EXTENDED -D_GNU_SOURCE -DHAVE_CONFIG_H -g -O2 -std=gnu99 -Wall -Werror=format-security -o cldr.build.o -c cldr.c
cldr.c:31:10: fatal error: expat.h: No such file or directory
 #include <expat.h>
          ^~~~~~~~~

To fix this issue, add host-expat dependency as well as patch so that
the *_FOR_BUILD variables are properly passed to the sub-configure
script that brltty runs to build the host utilities.

Fixes:
 - http://autobuild.buildroot.org/results/362cfb57e4a91a066493269d8078d931529ddf69

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: use a different, simpler solution.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>