Make locale_headers only when doing pregen and not headers

due to dependencies against sysnum.h

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

Makefile.in

@@ -134,9 +134,9 @@ headers_clean-y += sysdep_common_headers-clean
 headers-y += $(target-headers-sysdep)
 
 headers: include/bits/uClibc_config.h
-	$(Q)$(if $(UCLIBC_HAS_LOCALE),$(MAKE) -C extra/locale locale_headers)
 
 pregen: include/bits/sysnum.h headers
+	$(Q)$(if $(UCLIBC_HAS_LOCALE),$(MAKE) -C extra/locale locale_headers)
 
 include/bits/sysnum.h: $(top_srcdir)extra/scripts/gen_bits_syscall_h.sh
 	$(Q)$(INSTALL) -d $(@D)