i2c: Add string argument to static_assert()

Some compilers require a string argument.

Change-Id: I241ab3954aaac430804524a5557eab4047a213b4
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
diff --git a/i2c-internal.h b/i2c-internal.h
index eed788e..95d245c 100644
--- a/i2c-internal.h
+++ b/i2c-internal.h
@@ -6,7 +6,7 @@
 #include "libmctp-i2c.h"
 
 /* Limited by bytecount field */
-static_assert(I2C_BTU <= 254);
+static_assert(I2C_BTU <= 254, "I2C BTU is limited to 254");
 
 #ifndef MCTP_I2C_NEIGH_COUNT
 #define MCTP_I2C_NEIGH_COUNT 4