Fix implicit declaration of ipv4NetmaskToPrefixLength

Rationale for using extern:
---------------------------
The ipv4NetmaskToPrefixLength function is implemnted
in libnetutils/ifc_utils.c inside the android_system_core
repository[1].

In the lineage-16.0 branch[2]:
- ipv4NetmaskToPrefixLength is not declared in any
  headers in this repository.
- In the same repository, libnetutils/dhcpclient.c uses extern
  for the declaration of ipv4NetmaskToPrefixLength.

References:
-----------
[1]https://github.com/LineageOS/android_system_core
[2]At the time of writing it was at the following commit:
   e7f238619 healthd: make periodic battery status a debug message

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/data.c b/data.c
index 72f0187..5fedf92 100644
--- a/data.c
+++ b/data.c
@@ -28,6 +28,8 @@
 #include <samsung-ril.h>
 #include <utils.h>
 
+extern int ipv4NetmaskToPrefixLength(in_addr_t mask);
+
 int ipc2ril_gprs_fail_cause(unsigned char fail_cause)
 {
 	switch (fail_cause) {