misc: check for libdbi, mandatory library dependency

Otherwise, openbsc fails to compile:

  CC     db.o
db.c:29:21: error: dbi/dbi.h: File does not exists

openbsc/configure.in

@@ -49,6 +49,7 @@ PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 0.1.28)
 dnl checks for header files
 AC_HEADER_STDC
 AC_CHECK_HEADERS(dahdi/user.h,,AC_MSG_WARN(DAHDI input driver will not be built))
+AC_CHECK_HEADERS(dbi/dbd.h,,AC_MSG_ERROR(DBI library is not installed))
 
 
 dnl Checks for typedefs, structures and compiler characteristics