lib: lz4: define LZ4_decompress_safe_partial

Fixes the following compilation error:
../fs/erofs/decompressor.c:160:8: error: implicit declaration of function 'LZ4_decompress_safe_partial' [-Werror,-Wimplicit-function-declaration]
        ret = LZ4_decompress_safe_partial(src + inputmargin, out,

include/linux/lz4.h

@@ -296,7 +296,6 @@ int LZ4_decompress_fast(const char *source, char *dest, int originalSize);
 int LZ4_decompress_safe(const char *source, char *dest, int compressedSize,
 	int maxDecompressedSize);
 
-#if 0
 /**
  * LZ4_decompress_safe_partial() - Decompress a block of size 'compressedSize'
  *	at position 'source' into buffer 'dest'
@@ -322,9 +321,8 @@ int LZ4_decompress_safe(const char *source, char *dest, int compressedSize,
  *	or a negative result in case of error
  *
  */
-static int LZ4_decompress_safe_partial(const char *source, char *dest,
+int LZ4_decompress_safe_partial(const char *source, char *dest,
 	int compressedSize, int targetOutputSize, int maxDecompressedSize);
-#endif
 
 /*-************************************************************************
  *	LZ4 HC Compression

lib/lz4/lz4_decompress.c

@@ -691,16 +691,14 @@ int LZ4_decompress_safe(const char *source, char *dest,
 				      noDict, (BYTE *)dest, NULL, 0);
 }
 
-#if 0
-static int LZ4_decompress_safe_partial(const char *src, char *dst,
+int LZ4_decompress_safe_partial(const char *src, char *dst,
 	int compressedSize, int targetOutputSize, int dstCapacity)
 {
 	dstCapacity = min(targetOutputSize, dstCapacity);
 	return LZ4_decompress_generic(src, dst, compressedSize, dstCapacity,
 				      endOnInputSize, partial_decode,
 				      noDict, (BYTE *)dst, NULL, 0);
 }
-#endif
 
 int LZ4_decompress_fast(const char *source, char *dest, int originalSize)
 {