linux-user: Fix warnings caused by missing 'static' attribute

Warnings from the Sparse static analysis tool:

linux-user/main.c:40:12: warning:
 symbol 'filename' was not declared. Should it be static?
linux-user/main.c:41:12: warning:
 symbol 'argv0' was not declared. Should it be static?
linux-user/main.c:42:5: warning:
 symbol 'gdbstub_port' was not declared. Should it be static?
linux-user/main.c:43:11: warning:
 symbol 'envlist' was not declared. Should it be static?
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

linux-user/main.c

@@ -37,10 +37,10 @@
 char *exec_path;
 
 int singlestep;
-const char *filename;
-const char *argv0;
-int gdbstub_port;
-envlist_t *envlist;
+static const char *filename;
+static const char *argv0;
+static int gdbstub_port;
+static envlist_t *envlist;
 static const char *cpu_model;
 unsigned long mmap_min_addr;
 unsigned long guest_base;