.gitignore: add include/osmocom/core/socket_compat.h

Ensure that this auto-generated file does not show up in `git status`.

Change-Id: Id7c245579f5a3288a9ea62589ca802e427e54720
diff --git a/.gitignore b/.gitignore
index d900138..264e739 100644
--- a/.gitignore
+++ b/.gitignore
@@ -98,6 +98,7 @@
 src/gsm/*_gen.c
 include/osmocom/core/crc*gen.h
 include/osmocom/core/bit*gen.h
+include/osmocom/core/socket_compat.h
 include/osmocom/gsm/gsm0503.h
 tests/conv/gsm0503_test_vectors.c