Fix compilation when TRE_USE_SYSTEM_REGEX

tre.h was changed to define REG_USEBYTES if TRE_USE_SYSTEM_REGEX was off which broke the compilation step when it was on. This fixes the compilation by defining REG_USEBYTES.

Error message:

    make  all-recursive
    make[1]: Entering directory `/usr/share/media/src/git/tre'
    Making all in lib
    make[2]: Entering directory `/usr/share/media/src/git/tre/lib'
    make  all-am
    make[3]: Entering directory `/usr/share/media/src/git/tre/lib'
    /bin/sh ../libtool --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I..     -g -O2 -MT tre-ast.lo -MD -MP -MF .deps/tre-ast.Tpo -c -o tre-ast.lo tre-ast.c
    libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT tre-ast.lo -MD -MP -MF .deps/tre-ast.Tpo -c tre-ast.c  -fPIC -DPIC -o .libs/tre-ast.o
    mv -f .deps/tre-ast.Tpo .deps/tre-ast.Plo
    /bin/sh ../libtool --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I..     -g -O2 -MT tre-compile.lo -MD -MP -MF .deps/tre-compile.Tpo -c -o tre-compile.lo tre-compile.c
    libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT tre-compile.lo -MD -MP -MF .deps/tre-compile.Tpo -c tre-compile.c  -fPIC -DPIC -o .libs/tre-compile.o
    tre-compile.c: In function 'tre_compile':
    tre-compile.c:1894:33: error: 'REG_USEBYTES' undeclared (first use in this function)
       parse_ctx.cur_max = (cflags & REG_USEBYTES) ? 1 : TRE_MB_CUR_MAX;
                                     ^
    tre-compile.c:1894:33: note: each undeclared identifier is reported only once for each function it appears in
    make[3]: *** [tre-compile.lo] Error 1
    make[3]: Leaving directory `/usr/share/media/src/git/tre/lib'
    make[2]: *** [all] Error 2
    make[2]: Leaving directory `/usr/share/media/src/git/tre/lib'
    make[1]: *** [all-recursive] Error 1
    make[1]: Leaving directory `/usr/share/media/src/git/tre'
    make: *** [all] Error 2