Fix compile issue for PowerPC (#99)

There seems to be one opening parenthesis too many here on both lines.

Discovered after running into the following errors while cross-compiling for PowerPC/Qoriq:
```
src/_csrc/sha2.c: In function 'SHA256Final':
src/_csrc/sha2.c:484: error: expected ')' before ';' token
src/_csrc/sha2.c:485: error: expected ')' before '}' token
src/_csrc/sha2.c:485: error: expected ';' before '}' token
```

1 files changed, 2 insertions, 2 deletions

diff --git a/src/_csrc/portable_endian.h b/src/_csrc/portable_endian.h
index 848803b..4816331 100644
--- a/src/_csrc/portable_endian.h
+++ b/src/_csrc/portable_endian.h
@@ -48,9 +48,9 @@
 #           define le64toh(x) (x)
 #       elif defined(__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)
 #           define htobe16(x) (x)
-#           define htole16(x) ((((((uint16_t)(x)) >> 8))|((((uint16_t)(x)) << 8)))
+#           define htole16(x) (((((uint16_t)(x)) >> 8))|((((uint16_t)(x)) << 8)))
 #           define be16toh(x) (x)
-#           define le16toh(x) ((((((uint16_t)(x)) >> 8))|((((uint16_t)(x)) << 8)))
+#           define le16toh(x) (((((uint16_t)(x)) >> 8))|((((uint16_t)(x)) << 8)))
 
 #           define htobe32(x) (x)
 #           define htole32(x) (((uint32_t)htole16(((uint16_t)(((uint32_t)(x)) >> 16)))) | (((uint32_t)htole16(((uint16_t)(x)))) << 16))