random: Provide missing gettid() declaration

Provide missing declaration of gettid() function necessary for entropy
generation for hash table in the _rand_init_seed() function in the
src/libnm-glib-aux/nm-random-utils.c.

Fixes: 3649efe2b5 ('glib-aux: put more effort into seeding GRand fallback for nm_utils_random_bytes()')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/754

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/918
(cherry picked from commit e874c5bf6b)

src/libnm-glib-aux/nm-shared-utils.h

@@ -30,6 +30,16 @@ nm_is_ascii(char ch)
 
 /*****************************************************************************/
 
+#include <sys/syscall.h>
+static inline pid_t
+_nm_gettid(void)
+{
+    return (pid_t) syscall(SYS_gettid);
+}
+#define gettid() _nm_gettid()
+
+/*****************************************************************************/
+
 pid_t nm_utils_gettid(void);
 
 gboolean _nm_assert_on_main_thread(void);

src/libnm-systemd-shared/sd-adapt-shared/nm-sd-adapt-shared.h

@@ -75,13 +75,6 @@ raw_getpid(void)
     #endif
 }
 
-static inline pid_t
-_nm_gettid(void)
-{
-    return (pid_t) syscall(SYS_gettid);
-}
-    #define gettid() _nm_gettid()
-
     /* we build with C11 and thus <uchar.h> provides char32_t,char16_t. */
     #define HAVE_CHAR32_T 1
     #define HAVE_CHAR16_T 1