build: icc: silence some warnings

This disables the warning "external declaration in primary source file"
which is issued when a prototype for an extern function is found in a
.c file rather than a header file.  We have such prototypes for asm
functions where a separate header file would be pointless.

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -3129,10 +3129,11 @@ if enabled icc; then
     # -wd: Disable following warnings
     # 144, 167, 556: -Wno-pointer-sign
     # 1292: attribute "foo" ignored
+    # 1419: external declaration in primary source file
     # 10006: ignoring unknown option -fno-signed-zeros
     # 10148: ignoring unknown option -Wno-parentheses
     # 10156: ignoring option '-W'; no argument required
-    check_cflags -wd144,167,556,1292,10006,10148,10156
+    check_cflags -wd144,167,556,1292,1419,10006,10148,10156
     # 11030: Warning unknown option --as-needed
     # 10156: ignoring option '-export'; no argument required
     check_ldflags -wd10156,11030