lib: make LZ4_decompress_safe_forceExtDict() static

LZ4_decompress_safe_forceExtDict() is only used in
lib/lz4/lz4_decompress.c, make it static to fix the build warning about
"no previous prototype" [1].

[1] https://lore.kernel.org/lkml/[e-mail]/

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Tiezhu Yang <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/lz4/lz4_decompress.c

···
 				      (BYTE *)dest - prefixSize, NULL, 0);
 }
 
-int LZ4_decompress_safe_forceExtDict(const char *source, char *dest,
-				     int compressedSize, int maxOutputSize,
-				     const void *dictStart, size_t dictSize)
+static int LZ4_decompress_safe_forceExtDict(const char *source, char *dest,
+					    int compressedSize, int maxOutputSize,
+					    const void *dictStart, size_t dictSize)
 {
 	return LZ4_decompress_generic(source, dest,
 				      compressedSize, maxOutputSize,